PerlDoc perlvar

NAME
perlvar - Perl predefined variables

DESCRIPTION
The Syntax of Variable Names
Variable names in Perl can have several formats. Usually, they must
begin with a letter or underscore, in which case they can be arbitrarily
long (up to an internal limit of 251 characters) and may contain
letters, digits, underscores, or the special sequence "::" or "'". In
this case, the part before the last "::" or "'" is taken to be a
*package qualifier*; see perlmod.

Perl variable names may also be a sequence of digits or a single
punctuation or control character. These names are all reserved for
special uses by Perl; for example, the all-digits names are used to hold
data captured by backreferences after a regular expression match. Perl
has a special syntax for the single-control-character names: It
understands "^X" (caret "X") to mean the control-"X" character. For
example, the notation $^W (dollar-sign caret "W") is the scalar variable
whose name is the single character control-"W". This is better than
typing a literal control-"W" into your program.

Since Perl 5.6, Perl variable names may be alphanumeric strings that
begin with control characters (or better yet, a caret). These variables
must be written in the form "${^Foo}"; the braces are not optional.
"${^Foo}" denotes the scalar variable whose name is a control-"F"
followed by two "o"'s. These variables are reserved for future special
uses by Perl, except for the ones that begin with "^_"
(control-underscore or caret-underscore). No control-character name that
begins with "^_" will acquire a special meaning in any future version of
Perl; such names may therefore be used safely in programs. $^_ itself,
however, *is* reserved.

Perl identifiers that begin with digits, control characters, or
punctuation characters are exempt from the effects of the "package"
declaration and are always forced to be in package "main"; they are also
exempt from "strict 'vars'" errors. A few other names are also exempt in
these ways:

ENV STDIN
INC STDOUT
ARGV STDERR
ARGVOUT
SIG

In particular, the special "${^_XYZ}" variables are always taken to be
in package "main", regardless of any "package" declarations presently in
scope.

SPECIAL VARIABLES
The following names have special meaning to Perl. Most punctuation names
have reasonable mnemonics, or analogs in the shells. Nevertheless, if
you wish to use long variable names, you need only say:

use English;

at the top of your program. This aliases all the short names to the long
names in the current package. Some even have medium names, generally
borrowed from awk. To avoid a performance hit, if you don't need the
$PREMATCH, $MATCH, or $POSTMATCH it's best to use the "English" module
without them:

use English '-no_match_vars';

Before you continue, note the sort order for variables. In general, we
first list the variables in case-insensitive, almost-lexigraphical order
(ignoring the "{" or "^" preceding words, as in "${^UNICODE}" or $^T),
although $_ and @_ move up to the top of the pile. For variables with
the same identifier, we list it in order of scalar, array, hash, and
bareword.

General Variables
$ARG
$_ The default input and pattern-searching space. The following
pairs are equivalent:

while (<>) {...} # equivalent only in while!
while (defined($_ = <>)) {...}

/^Subject:/
$_ =~ /^Subject:/

tr/a-z/A-Z/
$_ =~ tr/a-z/A-Z/

chomp
chomp($_)

Here are the places where Perl will assume $_ even if you don't
use it:

* The following functions use $_ as a default argument:

abs, alarm, chomp, chop, chr, chroot, cos, defined, eval,
evalbytes, exp, glob, hex, int, lc, lcfirst, length, log,
lstat, mkdir, oct, ord, pos, print, quotemeta, readlink,
readpipe, ref, require, reverse (in scalar context only),
rmdir, sin, split (on its second argument), sqrt, stat,
study, uc, ucfirst, unlink, unpack.

* All file tests ("-f", "-d") except for "-t", which defaults
to STDIN. See "-X" in perlfunc

* The pattern matching operations "m//", "s///" and "tr///"
(aka "y///") when used without an "=~" operator.

* The default iterator variable in a "foreach" loop if no other
variable is supplied.

* The implicit iterator variable in the "grep()" and "map()"
functions.

* The implicit variable of "given()".

* The default place to put an input record when a "<FH>"
operation's result is tested by itself as the sole criterion
of a "while" test. Outside a "while" test, this will not
happen.

As $_ is a global variable, this may lead in some cases to
unwanted side-effects. As of perl 5.10, you can now use a
lexical version of $_ by declaring it in a file or in a block
with "my". Moreover, declaring "our $_" restores the global $_
in the current scope.

Mnemonic: underline is understood in certain operations.

@ARG
@_ Within a subroutine the array @_ contains the parameters passed
to that subroutine. Inside a subroutine, @_ is the default array
for the array operators "push", "pop", "shift", and "unshift".

See perlsub.

$LIST_SEPARATOR
$" When an array or an array slice is interpolated into a
double-quoted string or a similar context such as "/.../", its
elements are separated by this value. Default is a space. For
example, this:

print "The array is: @array\n";

is equivalent to this:

print "The array is: " . join($", @array) . "\n";

Mnemonic: works in double-quoted context.

$PROCESS_ID
$PID
$$ The process number of the Perl running this script. Though you
*can* set this variable, doing so is generally discouraged,
although it can be invaluable for some testing purposes. It will
be reset automatically across "fork()" calls.

Note for Linux and Debian GNU/kFreeBSD users: Before Perl
v5.16.0 perl would emulate POSIX semantics on Linux systems
using LinuxThreads, a partial implementation of POSIX Threads
that has since been superseded by the Native POSIX Thread
Library (NPTL).

LinuxThreads is now obsolete on Linux, and and caching
"getpid()" like this made embedding perl unnecessarily complex
(since you'd have to manually update the value of $$), so now $$
and "getppid()" will always return the same values as the
underlying C library.

Debian GNU/kFreeBSD systems also used LinuxThreads up until and
including the 6.0 release, but after that moved to FreeBSD
thread semantics, which are POSIX-like.

To see if your system is affected by this discrepancy check if
"getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL" returns a false
value. NTPL threads preserve the POSIX semantics.

Mnemonic: same as shells.

$PROGRAM_NAME
$0 Contains the name of the program being executed.

On some (but not all) operating systems assigning to $0 modifies
the argument area that the "ps" program sees. On some platforms
you may have to use special "ps" options or a different "ps" to
see the changes. Modifying the $0 is more useful as a way of
indicating the current program state than it is for hiding the
program you're running.

Note that there are platform-specific limitations on the maximum
length of $0. In the most extreme case it may be limited to the
space occupied by the original $0.

In some platforms there may be arbitrary amount of padding, for
example space characters, after the modified name as shown by
"ps". In some platforms this padding may extend all the way to
the original length of the argument area, no matter what you do
(this is the case for example with Linux 2.2).

Note for BSD users: setting $0 does not completely remove "perl"
from the ps(1) output. For example, setting $0 to "foobar" may
result in "perl: foobar (perl)" (whether both the "perl: "
prefix and the " (perl)" suffix are shown depends on your exact
BSD variant and version). This is an operating system feature,
Perl cannot help it.

In multithreaded scripts Perl coordinates the threads so that
any thread may modify its copy of the $0 and the change becomes
visible to ps(1) (assuming the operating system plays along).
Note that the view of $0 the other threads have will not change
since they have their own copies of it.

If the program has been given to perl via the switches "-e" or
"-E", $0 will contain the string "-e".

On Linux as of perl 5.14 the legacy process name will be set
with prctl(2), in addition to altering the POSIX name via
"argv[0]" as perl has done since version 4.000. Now system
utilities that read the legacy process name such as ps, top and
killall will recognize the name you set when assigning to $0.
The string you supply will be cut off at 16 bytes, this is a
limitation imposed by Linux.

Mnemonic: same as sh and ksh.

$REAL_GROUP_ID
$GID
$( The real gid of this process. If you are on a machine that
supports membership in multiple groups simultaneously, gives a
space separated list of groups you are in. The first number is
the one returned by "getgid()", and the subsequent ones by
"getgroups()", one of which may be the same as the first number.

However, a value assigned to $( must be a single number used to
set the real gid. So the value given by $( should *not* be
assigned back to $( without being forced numeric, such as by
adding zero. Note that this is different to the effective gid
($)) which does take a list.

You can change both the real gid and the effective gid at the
same time by using "POSIX::setgid()". Changes to $( require a
check to $! to detect any possible errors after an attempted
change.

Mnemonic: parentheses are used to *group* things. The real gid
is the group you *left*, if you're running setgid.

$EFFECTIVE_GROUP_ID
$EGID
$) The effective gid of this process. If you are on a machine that
supports membership in multiple groups simultaneously, gives a
space separated list of groups you are in. The first number is
the one returned by "getegid()", and the subsequent ones by
"getgroups()", one of which may be the same as the first number.

Similarly, a value assigned to $) must also be a space-separated
list of numbers. The first number sets the effective gid, and
the rest (if any) are passed to "setgroups()". To get the effect
of an empty list for "setgroups()", just repeat the new
effective gid; that is, to force an effective gid of 5 and an
effectively empty "setgroups()" list, say " $) = "5 5" ".

You can change both the effective gid and the real gid at the
same time by using "POSIX::setgid()" (use only a single numeric
argument). Changes to $) require a check to $! to detect any
possible errors after an attempted change.

$<, $>, $( and $) can be set only on machines that support the
corresponding *set[re][ug]id()* routine. $( and $) can be
swapped only on machines supporting "setregid()".

Mnemonic: parentheses are used to *group* things. The effective
gid is the group that's *right* for you, if you're running
setgid.

$REAL_USER_ID
$UID
$< The real uid of this process. You can change both the real uid
and the effective uid at the same time by using
"POSIX::setuid()". Since changes to $< require a system call,
check $! after a change attempt to detect any possible errors.

Mnemonic: it's the uid you came *from*, if you're running
setuid.

$EFFECTIVE_USER_ID
$EUID
$> The effective uid of this process. For example:

$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uids

You can change both the effective uid and the real uid at the
same time by using "POSIX::setuid()". Changes to $> require a
check to $! to detect any possible errors after an attempted
change.

$< and $> can be swapped only on machines supporting
"setreuid()".

Mnemonic: it's the uid you went *to*, if you're running setuid.

$SUBSCRIPT_SEPARATOR
$SUBSEP
$; The subscript separator for multidimensional array emulation. If
you refer to a hash element as

$foo{$a,$b,$c}

it really means

$foo{join($;, $a, $b, $c)}

But don't put

@foo{$a,$b,$c} # a slice--note the @

which means

($foo{$a},$foo{$b},$foo{$c})

Default is "\034", the same as SUBSEP in awk. If your keys
contain binary data there might not be any safe value for $;.

Consider using "real" multidimensional arrays as described in
perllol.

Mnemonic: comma (the syntactic subscript separator) is a
semi-semicolon.

$a
$b Special package variables when using "sort()", see "sort" in
perlfunc. Because of this specialness $a and $b don't need to be
declared (using "use vars", or "our()") even when using the
"strict 'vars'" pragma. Don't lexicalize them with "my $a" or
"my $b" if you want to be able to use them in the "sort()"
comparison block or function.

%ENV The hash %ENV contains your current environment. Setting a value
in "ENV" changes the environment for any child processes you
subsequently "fork()" off.

$SYSTEM_FD_MAX
$^F The maximum system file descriptor, ordinarily 2. System file
descriptors are passed to "exec()"ed processes, while higher
file descriptors are not. Also, during an "open()", system file
descriptors are preserved even if the "open()" fails (ordinary
file descriptors are closed before the "open()" is attempted).
The close-on-exec status of a file descriptor will be decided
according to the value of $^F when the corresponding file, pipe,
or socket was opened, not the time of the "exec()".

@F The array @F contains the fields of each line read in when
autosplit mode is turned on. See perlrun for the -a switch. This
array is package-specific, and must be declared or given a full
package name if not in package main when running under "strict
'vars'".

@INC The array @INC contains the list of places that the "do EXPR",
"require", or "use" constructs look for their library files. It
initially consists of the arguments to any -I command-line
switches, followed by the default Perl library, probably
/usr/local/lib/perl, followed by ".", to represent the current
directory. ("." will not be appended if taint checks are
enabled, either by "-T" or by "-t".) If you need to modify this
at runtime, you should use the "use lib" pragma to get the
machine-dependent library properly loaded also:

use lib '/mypath/libdir/';
use SomeMod;

You can also insert hooks into the file inclusion system by
putting Perl code directly into @INC. Those hooks may be
subroutine references, array references or blessed objects. See
"require" in perlfunc for details.

%INC The hash %INC contains entries for each filename included via
the "do", "require", or "use" operators. The key is the filename
you specified (with module names converted to pathnames), and
the value is the location of the file found. The "require"
operator uses this hash to determine whether a particular file
has already been included.

If the file was loaded via a hook (e.g. a subroutine reference,
see "require" in perlfunc for a description of these hooks),
this hook is by default inserted into %INC in place of a
filename. Note, however, that the hook may have set the %INC
entry by itself to provide some more specific info.

$INPLACE_EDIT
$^I The current value of the inplace-edit extension. Use "undef" to
disable inplace editing.

Mnemonic: value of -i switch.

$^M By default, running out of memory is an untrappable, fatal
error. However, if suitably built, Perl can use the contents of
$^M as an emergency memory pool after "die()"ing. Suppose that
your Perl were compiled with "-DPERL_EMERGENCY_SBRK" and used
Perl's malloc. Then

$^M = 'a' x (1 << 16);

would allocate a 64K buffer for use in an emergency. See the
INSTALL file in the Perl distribution for information on how to
add custom C compilation flags when compiling perl. To
discourage casual use of this advanced feature, there is no
English long name for this variable.

This variable was added in Perl 5.004.

$OSNAME
$^O The name of the operating system under which this copy of Perl
was built, as determined during the configuration process. For
examples see "PLATFORMS" in perlport.

The value is identical to $Config{'osname'}. See also Config and
the -V command-line switch documented in perlrun.

In Windows platforms, $^O is not very helpful: since it is
always "MSWin32", it doesn't tell the difference between
95/98/ME/NT/2000/XP/CE/.NET. Use "Win32::GetOSName()" or
Win32::GetOSVersion() (see Win32 and perlport) to distinguish
between the variants.

This variable was added in Perl 5.003.

%SIG The hash %SIG contains signal handlers for signals. For example:

sub handler { # 1st argument is signal name
my($sig) = @_;
print "Caught a SIG$sig--shutting down\n";
close(LOG);
exit(0);
}

$SIG{'INT'} = \&handler;
$SIG{'QUIT'} = \&handler;
...
$SIG{'INT'} = 'DEFAULT'; # restore default action
$SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT

Using a value of 'IGNORE' usually has the effect of ignoring the
signal, except for the "CHLD" signal. See perlipc for more about
this special case.

Here are some other examples:

$SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not
# recommended)
$SIG{"PIPE"} = \&Plumber; # just fine; assume current
# Plumber
$SIG{"PIPE"} = *Plumber; # somewhat esoteric
$SIG{"PIPE"} = Plumber(); # oops, what did Plumber()
# return??

Be sure not to use a bareword as the name of a signal handler,
lest you inadvertently call it.

If your system has the "sigaction()" function then signal
handlers are installed using it. This means you get reliable
signal handling.

The default delivery policy of signals changed in Perl 5.8.0
from immediate (also known as "unsafe") to deferred, also known
as "safe signals". See perlipc for more information.

Certain internal hooks can be also set using the %SIG hash. The
routine indicated by $SIG{__WARN__} is called when a warning
message is about to be printed. The warning message is passed as
the first argument. The presence of a "__WARN__" hook causes the
ordinary printing of warnings to "STDERR" to be suppressed. You
can use this to save warnings in a variable, or turn warnings
into fatal errors, like this:

local $SIG{__WARN__} = sub { die $_[0] };
eval $proggie;

As the 'IGNORE' hook is not supported by "__WARN__", you can
disable warnings using the empty subroutine:

local $SIG{__WARN__} = sub {};

The routine indicated by $SIG{__DIE__} is called when a fatal
exception is about to be thrown. The error message is passed as
the first argument. When a "__DIE__" hook routine returns, the
exception processing continues as it would have in the absence
of the hook, unless the hook routine itself exits via a "goto
&sub", a loop exit, or a "die()". The "__DIE__" handler is
explicitly disabled during the call, so that you can die from a
"__DIE__" handler. Similarly for "__WARN__".

Due to an implementation glitch, the $SIG{__DIE__} hook is
called even inside an "eval()". Do not use this to rewrite a
pending exception in $@, or as a bizarre substitute for
overriding "CORE::GLOBAL::die()". This strange action at a
distance may be fixed in a future release so that $SIG{__DIE__}
is only called if your program is about to exit, as was the
original intent. Any other use is deprecated.

"__DIE__"/"__WARN__" handlers are very special in one respect:
they may be called to report (probable) errors found by the
parser. In such a case the parser may be in inconsistent state,
so any attempt to evaluate Perl code from such a handler will
probably result in a segfault. This means that warnings or
errors that result from parsing Perl should be used with extreme
caution, like this:

require Carp if defined $^S;
Carp::confess("Something wrong") if defined &Carp::confess;
die "Something wrong, but could not load Carp to give "
. "backtrace...\n\t"
. "To see backtrace try starting Perl with -MCarp switch";

Here the first line will load "Carp" *unless* it is the parser
who called the handler. The second line will print backtrace and
die if "Carp" was available. The third line will be executed
only if "Carp" was not available.

Having to even think about the $^S variable in your exception
handlers is simply wrong. $SIG{__DIE__} as currently implemented
invites grievous and difficult to track down errors. Avoid it
and use an "END{}" or CORE::GLOBAL::die override instead.

See "die" in perlfunc, "warn" in perlfunc, "eval" in perlfunc,
and warnings for additional information.

$BASETIME
$^T The time at which the program began running, in seconds since
the epoch (beginning of 1970). The values returned by the -M,
-A, and -C filetests are based on this value.

$PERL_VERSION
$^V The revision, version, and subversion of the Perl interpreter,
represented as a "version" object.

This variable first appeared in perl 5.6.0; earlier versions of
perl will see an undefined value. Before perl 5.10.0 $^V was
represented as a v-string.

$^V can be used to determine whether the Perl interpreter
executing a script is in the right range of versions. For
example:

warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1

To convert $^V into its string representation use "sprintf()"'s
"%vd" conversion:

printf "version is v%vd\n", $^V; # Perl's version

See the documentation of "use VERSION" and "require VERSION" for
a convenient way to fail if the running Perl interpreter is too
old.

See also $] for an older representation of the Perl version.

This variable was added in Perl 5.6.

Mnemonic: use ^V for Version Control.

${^WIN32_SLOPPY_STAT}
If this variable is set to a true value, then "stat()" on
Windows will not try to open the file. This means that the link
count cannot be determined and file attributes may be out of
date if additional hardlinks to the file exist. On the other
hand, not opening the file is considerably faster, especially
for files on network drives.

This variable could be set in the sitecustomize.pl file to
configure the local Perl installation to use "sloppy" "stat()"
by default. See the documentation for -f in perlrun for more
information about site customization.

This variable was added in Perl 5.10.

$EXECUTABLE_NAME
$^X The name used to execute the current copy of Perl, from C's
"argv[0]" or (where supported) /proc/self/exe.

Depending on the host operating system, the value of $^X may be
a relative or absolute pathname of the perl program file, or may
be the string used to invoke perl but not the pathname of the
perl program file. Also, most operating systems permit invoking
programs that are not in the PATH environment variable, so there
is no guarantee that the value of $^X is in PATH. For VMS, the
value may or may not include a version number.

You usually can use the value of $^X to re-invoke an independent
copy of the same perl that is currently running, e.g.,

@first_run = `$^X -le "print int rand 100 for 1..100"`;

But recall that not all operating systems support forking or
capturing of the output of commands, so this complex statement
may not be portable.

It is not safe to use the value of $^X as a path name of a file,
as some operating systems that have a mandatory suffix on
executable files do not require use of the suffix when invoking
a command. To convert the value of $^X to a path name, use the
following statements:

# Build up a set of file names (not command names).
use Config;
my $this_perl = $^X;
if ($^O ne 'VMS') {
$this_perl .= $Config{_exe}
unless $this_perl =~ m/$Config{_exe}$/i;
}

Because many operating systems permit anyone with read access to
the Perl program file to make a copy of it, patch the copy, and
then execute the copy, the security-conscious Perl programmer
should take care to invoke the installed copy of perl, not the
copy referenced by $^X. The following statements accomplish this
goal, and produce a pathname that can be invoked as a command or
referenced as a file.

use Config;
my $secure_perl_path = $Config{perlpath};
if ($^O ne 'VMS') {
$secure_perl_path .= $Config{_exe}
unless $secure_perl_path =~ m/$Config{_exe}$/i;
}

Variables related to regular expressions
Most of the special variables related to regular expressions are side
effects. Perl sets these variables when it has a successful match, so
you should check the match result before using them. For instance:

if( /P(A)TT(ER)N/ ) {
print "I found $1 and $2\n";
}

These variables are read-only and dynamically-scoped, unless we note
otherwise.

The dynamic nature of the regular expression variables means that their
value is limited to the block that they are in, as demonstrated by this
bit of code:

my $outer = 'Wallace and Grommit';
my $inner = 'Mutt and Jeff';

my $pattern = qr/(\S+) and (\S+)/;

sub show_n { print "\$1 is $1; \$2 is $2\n" }

{
OUTER:
show_n() if $outer =~ m/$pattern/;

INNER: {
show_n() if $inner =~ m/$pattern/;
}

show_n();
}

The output shows that while in the "OUTER" block, the values of $1 and
$2 are from the match against $outer. Inside the "INNER" block, the
values of $1 and $2 are from the match against $inner, but only until
the end of the block (i.e. the dynamic scope). After the "INNER" block
completes, the values of $1 and $2 return to the values for the match
against $outer even though we have not made another match:

$1 is Wallace; $2 is Grommit
$1 is Mutt; $2 is Jeff
$1 is Wallace; $2 is Grommit

Due to an unfortunate accident of Perl's implementation, "use English"
imposes a considerable performance penalty on all regular expression
matches in a program because it uses the $`, $&, and $', regardless of
whether they occur in the scope of "use English". For that reason,
saying "use English" in libraries is strongly discouraged unless you
import it without the match variables:

use English '-no_match_vars'

The "Devel::NYTProf" and "Devel::FindAmpersand" modules can help you
find uses of these problematic match variables in your code.

Since Perl 5.10, you can use the "/p" match operator flag and the
"${^PREMATCH}", "${^MATCH}", and "${^POSTMATCH}" variables instead so
you only suffer the performance penalties.

$<*digits*> ($1, $2, ...)
Contains the subpattern from the corresponding set of capturing
parentheses from the last successful pattern match, not counting
patterns matched in nested blocks that have been exited already.

These variables are read-only and dynamically-scoped.

Mnemonic: like \digits.

$MATCH
$& The string matched by the last successful pattern match (not
counting any matches hidden within a BLOCK or "eval()" enclosed
by the current BLOCK).

The use of this variable anywhere in a program imposes a
considerable performance penalty on all regular expression
matches. To avoid this penalty, you can extract the same
substring by using "@-". Starting with Perl 5.10, you can use
the "/p" match flag and the "${^MATCH}" variable to do the same
thing for particular match operations.

This variable is read-only and dynamically-scoped.

Mnemonic: like "&" in some editors.

${^MATCH}
This is similar to $& ($MATCH) except that it does not incur the
performance penalty associated with that variable, and is only
guaranteed to return a defined value when the pattern was
compiled or executed with the "/p" modifier.

This variable was added in Perl 5.10.

This variable is read-only and dynamically-scoped.

$PREMATCH
$` The string preceding whatever was matched by the last successful
pattern match, not counting any matches hidden within a BLOCK or
"eval" enclosed by the current BLOCK.

The use of this variable anywhere in a program imposes a
considerable performance penalty on all regular expression
matches. To avoid this penalty, you can extract the same
substring by using "@-". Starting with Perl 5.10, you can use
the "/p" match flag and the "${^PREMATCH}" variable to do the
same thing for particular match operations.

This variable is read-only and dynamically-scoped.

Mnemonic: "`" often precedes a quoted string.

${^PREMATCH}
This is similar to $` ($PREMATCH) except that it does not incur
the performance penalty associated with that variable, and is
only guaranteed to return a defined value when the pattern was
compiled or executed with the "/p" modifier.

This variable was added in Perl 5.10

This variable is read-only and dynamically-scoped.

$POSTMATCH
$' The string following whatever was matched by the last successful
pattern match (not counting any matches hidden within a BLOCK or
"eval()" enclosed by the current BLOCK). Example:

local $_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi

The use of this variable anywhere in a program imposes a
considerable performance penalty on all regular expression
matches. To avoid this penalty, you can extract the same
substring by using "@-". Starting with Perl 5.10, you can use
the "/p" match flag and the "${^POSTMATCH}" variable to do the
same thing for particular match operations.

This variable is read-only and dynamically-scoped.

Mnemonic: "'" often follows a quoted string.

${^POSTMATCH}
This is similar to $' ($POSTMATCH) except that it does not incur
the performance penalty associated with that variable, and is
only guaranteed to return a defined value when the pattern was
compiled or executed with the "/p" modifier.

This variable was added in Perl 5.10.

This variable is read-only and dynamically-scoped.

$LAST_PAREN_MATCH
$+ The text matched by the last bracket of the last successful
search pattern. This is useful if you don't know which one of a
set of alternative patterns matched. For example:

/Version: (.*)|Revision: (.*)/ && ($rev = $+);

This variable is read-only and dynamically-scoped.

Mnemonic: be positive and forward looking.

$LAST_SUBMATCH_RESULT
$^N The text matched by the used group most-recently closed (i.e.
the group with the rightmost closing parenthesis) of the last
successful search pattern.

This is primarily used inside "(?{...})" blocks for examining
text recently matched. For example, to effectively capture text
to a variable (in addition to $1, $2, etc.), replace "(...)"
with

(?:(...)(?{ $var = $^N }))

By setting and then using $var in this way relieves you from
having to worry about exactly which numbered set of parentheses
they are.

This variable was added in Perl 5.8.

Mnemonic: the (possibly) Nested parenthesis that most recently
closed.

@LAST_MATCH_END
@+ This array holds the offsets of the ends of the last successful
submatches in the currently active dynamic scope. $+[0] is the
offset into the string of the end of the entire match. This is
the same value as what the "pos" function returns when called on
the variable that was matched against. The *n*th element of this
array holds the offset of the *n*th submatch, so $+[1] is the
offset past where $1 ends, $+[2] the offset past where $2 ends,
and so on. You can use $#+ to determine how many subgroups were
in the last successful match. See the examples given for the
"@-" variable.

This variable was added in Perl 5.6.

%LAST_PAREN_MATCH
%+ Similar to "@+", the "%+" hash allows access to the named
capture buffers, should they exist, in the last successful match
in the currently active dynamic scope.

For example, $+{foo} is equivalent to $1 after the following
match:

'foo' =~ /(?<foo>foo)/;

The keys of the "%+" hash list only the names of buffers that
have captured (and that are thus associated to defined values).

The underlying behaviour of "%+" is provided by the
Tie::Hash::NamedCapture module.

Note: "%-" and "%+" are tied views into a common internal hash
associated with the last successful regular expression.
Therefore mixing iterative access to them via "each" may have
unpredictable results. Likewise, if the last successful match
changes, then the results may be surprising.

This variable was added in Perl 5.10.

This variable is read-only and dynamically-scoped.

@LAST_MATCH_START
@- $-[0] is the offset of the start of the last successful match.
"$-["*n*"]" is the offset of the start of the substring matched
by *n*-th subpattern, or undef if the subpattern did not match.

Thus, after a match against $_, $& coincides with "substr $_,
$-[0], $+[0] - $-[0]". Similarly, $*n* coincides with "substr
$_, $-[n], $+[n] - $-[n]" if $-[n] is defined, and $+ coincides
with "substr $_, $-[$#-], $+[$#-] - $-[$#-]". One can use $#- to
find the last matched subgroup in the last successful match.
Contrast with $#+, the number of subgroups in the regular
expression. Compare with "@+".

This array holds the offsets of the beginnings of the last
successful submatches in the currently active dynamic scope.
$-[0] is the offset into the string of the beginning of the
entire match. The *n*th element of this array holds the offset
of the *n*th submatch, so $-[1] is the offset where $1 begins,
$-[2] the offset where $2 begins, and so on.

After a match against some variable $var:

$` is the same as "substr($var, 0, $-[0])"
$& is the same as "substr($var, $-[0], $+[0] - $-[0])"
$' is the same as "substr($var, $+[0])"
$1 is the same as "substr($var, $-[1], $+[1] - $-[1])"
$2 is the same as "substr($var, $-[2], $+[2] - $-[2])"
$3 is the same as "substr($var, $-[3], $+[3] - $-[3])"

This variable was added in Perl 5.6.

%LAST_MATCH_START
%- Similar to "%+", this variable allows access to the named
capture groups in the last successful match in the currently
active dynamic scope. To each capture group name found in the
regular expression, it associates a reference to an array
containing the list of values captured by all buffers with that
name (should there be several of them), in the order where they
appear.

Here's an example:

if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
foreach my $bufname (sort keys %-) {
my $ary = $-{$bufname};
foreach my $idx (0..$#$ary) {
print "\$-{$bufname}[$idx] : ",
(defined($ary->[$idx])
? "'$ary->[$idx]'"
: "undef"),
"\n";
}
}
}

would print out:

$-{A}[0] : '1'
$-{A}[1] : '3'
$-{B}[0] : '2'
$-{B}[1] : '4'

The keys of the "%-" hash correspond to all buffer names found
in the regular expression.

The behaviour of "%-" is implemented via the
Tie::Hash::NamedCapture module.

Note: "%-" and "%+" are tied views into a common internal hash
associated with the last successful regular expression.
Therefore mixing iterative access to them via "each" may have
unpredictable results. Likewise, if the last successful match
changes, then the results may be surprising.

This variable was added in Perl 5.10

This variable is read-only and dynamically-scoped.

$LAST_REGEXP_CODE_RESULT
$^R The result of evaluation of the last successful "(?{ code })"
regular expression assertion (see perlre). May be written to.

This variable was added in Perl 5.005.

${^RE_DEBUG_FLAGS}
The current value of the regex debugging flags. Set to 0 for no
debug output even when the "re 'debug'" module is loaded. See re
for details.

This variable was added in Perl 5.10.

${^RE_TRIE_MAXBUF}
Controls how certain regex optimisations are applied and how
much memory they utilize. This value by default is 65536 which
corresponds to a 512kB temporary cache. Set this to a higher
value to trade memory for speed when matching large
alternations. Set it to a lower value if you want the
optimisations to be as conservative of memory as possible but
still occur, and set it to a negative value to prevent the
optimisation and conserve the most memory. Under normal
situations this variable should be of no interest to you.

This variable was added in Perl 5.10.

Variables related to filehandles
Variables that depend on the currently selected filehandle may be set by
calling an appropriate object method on the "IO::Handle" object,
although this is less efficient than using the regular built-in
variables. (Summary lines below for this contain the word HANDLE.) First
you must say

use IO::Handle;

after which you may use either

method HANDLE EXPR

or more safely,

HANDLE->method(EXPR)

Each method returns the old value of the "IO::Handle" attribute. The
methods each take an optional EXPR, which, if supplied, specifies the
new value for the "IO::Handle" attribute in question. If not supplied,
most methods do nothing to the current value--except for "autoflush()",
which will assume a 1 for you, just to be different.

Because loading in the "IO::Handle" class is an expensive operation, you
should learn how to use the regular built-in variables.

A few of these variables are considered "read-only". This means that if
you try to assign to this variable, either directly or indirectly
through a reference, you'll raise a run-time exception.

You should be very careful when modifying the default values of most
special variables described in this document. In most cases you want to
localize these variables before changing them, since if you don't, the
change may affect other modules which rely on the default values of the
special variables that you have changed. This is one of the correct ways
to read the whole file at once:

open my $fh, "<", "foo" or die $!;
local $/; # enable localized slurp mode
my $content = <$fh>;
close $fh;

But the following code is quite bad:

open my $fh, "<", "foo" or die $!;
undef $/; # enable slurp mode
my $content = <$fh>;
close $fh;

since some other module, may want to read data from some file in the
default "line mode", so if the code we have just presented has been
executed, the global value of $/ is now changed for any other code
running inside the same Perl interpreter.

Usually when a variable is localized you want to make sure that this
change affects the shortest scope possible. So unless you are already
inside some short "{}" block, you should create one yourself. For
example:

my $content = '';
open my $fh, "<", "foo" or die $!;
{
local $/;
$content = <$fh>;
}
close $fh;

Here is an example of how your own code can go broken:

for ( 1..3 ){
$\ = "\r\n";
nasty_break();
print "$_";
}

sub nasty_break {
$\ = "\f";
# do something with $_
}

You probably expect this code to print the equivalent of

"1\r\n2\r\n3\r\n"

but instead you get:

"1\f2\f3\f"

Why? Because "nasty_break()" modifies $\ without localizing it first.
The value you set in "nasty_break()" is still there when you return. The
fix is to add "local()" so the value doesn't leak out of
"nasty_break()":

local $\ = "\f";

It's easy to notice the problem in such a short example, but in more
complicated code you are looking for trouble if you don't localize
changes to the special variables.

$ARGV Contains the name of the current file when reading from "<>".

@ARGV The array @ARGV contains the command-line arguments intended for
the script. $#ARGV is generally the number of arguments minus
one, because $ARGV[0] is the first argument, *not* the program's
command name itself. See "$0" for the command name.

ARGV The special filehandle that iterates over command-line filenames
in @ARGV. Usually written as the null filehandle in the angle
operator "<>". Note that currently "ARGV" only has its magical
effect within the "<>" operator; elsewhere it is just a plain
filehandle corresponding to the last file opened by "<>". In
particular, passing "\*ARGV" as a parameter to a function that
expects a filehandle may not cause your function to
automatically read the contents of all the files in @ARGV.

ARGVOUT The special filehandle that points to the currently open output
file when doing edit-in-place processing with -i. Useful when
you have to do a lot of inserting and don't want to keep
modifying $_. See perlrun for the -i switch.

Handle->output_field_separator( EXPR )
$OUTPUT_FIELD_SEPARATOR
$OFS
$, The output field separator for the print operator. If defined,
this value is printed between each of print's arguments. Default
is "undef".

Mnemonic: what is printed when there is a "," in your print
statement.

HANDLE->input_line_number( EXPR )
$INPUT_LINE_NUMBER
$NR
$. Current line number for the last filehandle accessed.

Each filehandle in Perl counts the number of lines that have
been read from it. (Depending on the value of $/, Perl's idea of
what constitutes a line may not match yours.) When a line is
read from a filehandle (via "readline()" or "<>"), or when
"tell()" or "seek()" is called on it, $. becomes an alias to the
line counter for that filehandle.

You can adjust the counter by assigning to $., but this will not
actually move the seek pointer. *Localizing $. will not localize
the filehandle's line count*. Instead, it will localize perl's
notion of which filehandle $. is currently aliased to.

$. is reset when the filehandle is closed, but not when an open
filehandle is reopened without an intervening "close()". For
more details, see "I/O Operators" in perlop. Because "<>" never
does an explicit close, line numbers increase across "ARGV"
files (but see examples in "eof" in perlfunc).

You can also use "HANDLE->input_line_number(EXPR)" to access the
line counter for a given filehandle without having to worry
about which handle you last accessed.

Mnemonic: many programs use "." to mean the current line number.

HANDLE->input_record_separator( EXPR )
$INPUT_RECORD_SEPARATOR
$RS
$/ The input record separator, newline by default. This influences
Perl's idea of what a "line" is. Works like awk's RS variable,
including treating empty lines as a terminator if set to the
null string (an empty line cannot contain any spaces or tabs).
You may set it to a multi-character string to match a
multi-character terminator, or to "undef" to read through the
end of file. Setting it to "\n\n" means something slightly
different than setting to "", if the file contains consecutive
empty lines. Setting to "" will treat two or more consecutive
empty lines as a single empty line. Setting to "\n\n" will
blindly assume that the next input character belongs to the next
paragraph, even if it's a newline.

local $/; # enable "slurp" mode
local $_ = <FH>; # whole file now here
s/\n[ \t]+/ /g;

Remember: the value of $/ is a string, not a regex. awk has to
be better for something. :-)

Setting $/ to a reference to an integer, scalar containing an
integer, or scalar that's convertible to an integer will attempt
to read records instead of lines, with the maximum record size
being the referenced integer. So this:

local $/ = \32768; # or \"32768", or \$var_containing_32768
open my $fh, "<", $myfile or die $!;
local $_ = <$fh>;

will read a record of no more than 32768 bytes from FILE. If
you're not reading from a record-oriented file (or your OS
doesn't have record-oriented files), then you'll likely get a
full chunk of data with every read. If a record is larger than
the record size you've set, you'll get the record back in
pieces. Trying to set the record size to zero or less will cause
reading in the (rest of the) whole file.

On VMS only, record reads bypass PerlIO layers and any
associated buffering,so you must not mix record and non-record
reads on the same filehandle. Record mode mixes with line mode
only when the same buffering layer is in use for both modes.

If you perform a record read on a FILE with an encoding layer
such as ":encoding(latin1)" or ":utf8", you may get an invalid
string as a result, may leave the FILE positioned between
characters in the stream and may not be reading the number of
bytes from the underlying file that you specified. This
behaviour may change without warning in a future version of
perl.

See also "Newlines" in perlport. Also see "$.".

Mnemonic: / delimits line boundaries when quoting poetry.

Handle->output_record_separator( EXPR )
$OUTPUT_RECORD_SEPARATOR
$ORS
$\ The output record separator for the print operator. If defined,
this value is printed after the last of print's arguments.
Default is "undef".

Mnemonic: you set $\ instead of adding "\n" at the end of the
print. Also, it's just like $/, but it's what you get "back"
from Perl.

HANDLE->autoflush( EXPR )
$OUTPUT_AUTOFLUSH
$| If set to nonzero, forces a flush right away and after every
write or print on the currently selected output channel. Default
is 0 (regardless of whether the channel is really buffered by
the system or not; $| tells you only whether you've asked Perl
explicitly to flush after each write). STDOUT will typically be
line buffered if output is to the terminal and block buffered
otherwise. Setting this variable is useful primarily when you
are outputting to a pipe or socket, such as when you are running
a Perl program under rsh and want to see the output as it's
happening. This has no effect on input buffering. See "getc" in
perlfunc for that. See "select" in perlfunc on how to select the
output channel. See also IO::Handle.

Mnemonic: when you want your pipes to be piping hot.

Variables related to formats
The special variables for formats are a subset of those for filehandles.
See perlform for more information about Perl's formats.

$ACCUMULATOR
$^A The current value of the "write()" accumulator for "format()"
lines. A format contains "formline()" calls that put their
result into $^A. After calling its format, "write()" prints out
the contents of $^A and empties. So you never really see the
contents of $^A unless you call "formline()" yourself and then
look at it. See perlform and "formline PICTURE,LIST" in
perlfunc.

HANDLE->format_formfeed(EXPR)
$FORMAT_FORMFEED
$^L What formats output as a form feed. The default is "\f".

HANDLE->format_page_number(EXPR)
$FORMAT_PAGE_NUMBER
$% The current page number of the currently selected output
channel.

Mnemonic: "%" is page number in nroff.

HANDLE->format_lines_left(EXPR)
$FORMAT_LINES_LEFT
$- The number of lines left on the page of the currently selected
output channel.

Mnemonic: lines_on_page - lines_printed.

Handle->format_line_break_characters EXPR
$FORMAT_LINE_BREAK_CHARACTERS
$: The current set of characters after which a string may be broken
to fill continuation fields (starting with "^") in a format. The
default is " \n-", to break on a space, newline, or a hyphen.

Mnemonic: a "colon" in poetry is a part of a line.

HANDLE->format_lines_per_page(EXPR)
$FORMAT_LINES_PER_PAGE
$= The current page length (printable lines) of the currently
selected output channel. The default is 60.

Mnemonic: = has horizontal lines.

HANDLE->format_top_name(EXPR)
$FORMAT_TOP_NAME
$^ The name of the current top-of-page format for the currently
selected output channel. The default is the name of the
filehandle with "_TOP" appended. For example, the default format
top name for the "STDOUT" filehandle is "STDOUT_TOP".

Mnemonic: points to top of page.

HANDLE->format_name(EXPR)
$FORMAT_NAME
$~ The name of the current report format for the currently selected
output channel. The default format name is the same as the
filehandle name. For example, the default format name for the
"STDOUT" filehandle is just "STDOUT".

Mnemonic: brother to $^.

Error Variables
The variables $@, $!, $^E, and $? contain information about different
types of error conditions that may appear during execution of a Perl
program. The variables are shown ordered by the "distance" between the
subsystem which reported the error and the Perl process. They correspond
to errors detected by the Perl interpreter, C library, operating system,
or an external program, respectively.

To illustrate the differences between these variables, consider the
following Perl expression, which uses a single-quoted string. After
execution of this statement, perl may have set all four special error
variables:

eval q{
open my $pipe, "/cdrom/install |" or die $!;
my @res = <$pipe>;
close $pipe or die "bad pipe: $?, $!";
};

When perl executes the "eval()" expression, it translates the "open()",
"<PIPE>", and "close" calls in the C run-time library and thence to the
operating system kernel. perl sets $! to the C library's "errno" if one
of these calls fails.

$@ is set if the string to be "eval"-ed did not compile (this may happen
if "open" or "close" were imported with bad prototypes), or if Perl code
executed during evaluation "die()"d. In these cases the value of $@ is
the compile error, or the argument to "die" (which will interpolate $!
and $?). (See also Fatal, though.)

Under a few operating systems, $^E may contain a more verbose error
indicator, such as in this case, "CDROM tray not closed." Systems that
do not support extended error messages leave $^E the same as $!.

Finally, $? may be set to non-0 value if the external program
/cdrom/install fails. The upper eight bits reflect specific error
conditions encountered by the program (the program's "exit()" value).
The lower eight bits reflect mode of failure, like signal death and core
dump information. See wait(2) for details. In contrast to $! and $^E,
which are set only if error condition is detected, the variable $? is
set on each "wait" or pipe "close", overwriting the old value. This is
more like $@, which on every "eval()" is always set on failure and
cleared on success.

For more details, see the individual descriptions at $@, $!, $^E, and
$?.

${^CHILD_ERROR_NATIVE}
The native status returned by the last pipe close, backtick (``)
command, successful call to "wait()" or "waitpid()", or from the
"system()" operator. On POSIX-like systems this value can be
decoded with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG,
WIFSTOPPED, WSTOPSIG and WIFCONTINUED functions provided by the
POSIX module.

Under VMS this reflects the actual VMS exit status; i.e. it is
the same as $? when the pragma "use vmsish 'status'" is in
effect.

This variable was added in Perl 5.8.9.

$EXTENDED_OS_ERROR
$^E Error information specific to the current operating system. At
the moment, this differs from $! under only VMS, OS/2, and Win32
(and for MacPerl). On all other platforms, $^E is always just
the same as $!.

Under VMS, $^E provides the VMS status value from the last
system error. This is more specific information about the last
system error than that provided by $!. This is particularly
important when $! is set to EVMSERR.

Under OS/2, $^E is set to the error code of the last call to
OS/2 API either via CRT, or directly from perl.

Under Win32, $^E always returns the last error information
reported by the Win32 call "GetLastError()" which describes the
last error from within the Win32 API. Most Win32-specific code
will report errors via $^E. ANSI C and Unix-like calls set
"errno" and so most portable Perl code will report errors via
$!.

Caveats mentioned in the description of $! generally apply to
$^E, also.

This variable was added in Perl 5.003.

Mnemonic: Extra error explanation.

$EXCEPTIONS_BEING_CAUGHT
$^S Current state of the interpreter.

$^S State
--------- -------------------
undef Parsing module/eval
true (1) Executing an eval
false (0) Otherwise

The first state may happen in $SIG{__DIE__} and $SIG{__WARN__}
handlers.

This variable was added in Perl 5.004.

$WARNING
$^W The current value of the warning switch, initially true if -w
was used, false otherwise, but directly modifiable.

See also warnings.

Mnemonic: related to the -w switch.

${^WARNING_BITS}
The current set of warning checks enabled by the "use warnings"
pragma. It has the same scoping as the $^H and "%^H" variables.
The exact values are considered internal to the warnings pragma
and may change between versions of Perl.

This variable was added in Perl 5.6.

$OS_ERROR
$ERRNO
$! When referenced, $! retrieves the current value of the C "errno"
integer variable. If $! is assigned a numerical value, that
value is stored in "errno". When referenced as a string, $!
yields the system error string corresponding to "errno".

Many system or library calls set "errno" if they fail, to
indicate the cause of failure. They usually do not set "errno"
to zero if they succeed. This means "errno", hence $!, is
meaningful only *immediately* after a failure:

if (open my $fh, "<", $filename) {
# Here $! is meaningless.
...
}
else {
# ONLY here is $! meaningful.
...
# Already here $! might be meaningless.
}
# Since here we might have either success or failure,
# $! is meaningless.

Here, *meaningless* means that $! may be unrelated to the
outcome of the "open()" operator. Assignment to $! is similarly
ephemeral. It can be used immediately before invoking the
"die()" operator, to set the exit value, or to inspect the
system error string corresponding to error *n*, or to restore $!
to a meaningful state.

Mnemonic: What just went bang?

%OS_ERROR
%ERRNO
%! Each element of "%!" has a true value only if $! is set to that
value. For example, $!{ENOENT} is true if and only if the
current value of $! is "ENOENT"; that is, if the most recent
error was "No such file or directory" (or its moral equivalent:
not all operating systems give that exact error, and certainly
not all languages). To check if a particular key is meaningful
on your system, use "exists $!{the_key}"; for a list of legal
keys, use "keys %!". See Errno for more information, and also
see "$!".

This variable was added in Perl 5.005.

$CHILD_ERROR
$? The status returned by the last pipe close, backtick (``)
command, successful call to "wait()" or "waitpid()", or from the
"system()" operator. This is just the 16-bit status word
returned by the traditional Unix "wait()" system call (or else
is made up to look like it). Thus, the exit value of the
subprocess is really ("$? >> 8"), and "$? & 127" gives which
signal, if any, the process died from, and "$? & 128" reports
whether there was a core dump.

Additionally, if the "h_errno" variable is supported in C, its
value is returned via $? if any "gethost*()" function fails.

If you have installed a signal handler for "SIGCHLD", the value
of $? will usually be wrong outside that handler.

Inside an "END" subroutine $? contains the value that is going
to be given to "exit()". You can modify $? in an "END"
subroutine to change the exit status of your program. For
example:

END {
$? = 1 if $? == 255; # die would make it 255
}

Under VMS, the pragma "use vmsish 'status'" makes $? reflect the
actual VMS exit status, instead of the default emulation of
POSIX status; see "$?" in perlvms for details.

Mnemonic: similar to sh and ksh.

$EVAL_ERROR
$@ The Perl syntax error message from the last "eval()" operator.
If $@ is the null string, the last "eval()" parsed and executed
correctly (although the operations you invoked may have failed
in the normal fashion).

Warning messages are not collected in this variable. You can,
however, set up a routine to process warnings by setting
$SIG{__WARN__} as described in "%SIG".

Mnemonic: Where was the syntax error "at"?

Variables related to the interpreter state
These variables provide information about the current interpreter state.

$COMPILING
$^C The current value of the flag associated with the -c switch.
Mainly of use with -MO=... to allow code to alter its behavior
when being compiled, such as for example to "AUTOLOAD" at
compile time rather than normal, deferred loading. Setting "$^C
= 1" is similar to calling "B::minus_c".

This variable was added in Perl 5.6.

$DEBUGGING
$^D The current value of the debugging flags. May be read or set.
Like its command-line equivalent, you can use numeric or
symbolic values, eg "$^D = 10" or "$^D = "st"".

Mnemonic: value of -D switch.

${^ENCODING}
The *object reference* to the "Encode" object that is used to
convert the source code to Unicode. Thanks to this variable your
Perl script does not have to be written in UTF-8. Default is
*undef*. The direct manipulation of this variable is highly
discouraged.

This variable was added in Perl 5.8.2.

${^GLOBAL_PHASE}
The current phase of the perl interpreter.

Possible values are:

CONSTRUCT
The "PerlInterpreter*" is being constructed via
"perl_construct". This value is mostly there for
completeness and for use via the underlying C variable
"PL_phase". It's not really possible for Perl code to be
executed unless construction of the interpreter is
finished.

START This is the global compile-time. That includes,
basically, every "BEGIN" block executed directly or
indirectly from during the compile-time of the top-level
program.

This phase is not called "BEGIN" to avoid confusion with
"BEGIN"-blocks, as those are executed during
compile-time of any compilation unit, not just the
top-level program. A new, localised compile-time entered
at run-time, for example by constructs as "eval "use
SomeModule"" are not global interpreter phases, and
therefore aren't reflected by "${^GLOBAL_PHASE}".

CHECK Execution of any "CHECK" blocks.

INIT Similar to "CHECK", but for "INIT"-blocks, not "CHECK"
blocks.

RUN The main run-time, i.e. the execution of "PL_main_root".

END Execution of any "END" blocks.

DESTRUCT
Global destruction.

Also note that there's no value for UNITCHECK-blocks. That's
because those are run for each compilation unit individually,
and therefore is not a global interpreter phase.

Not every program has to go through each of the possible phases,
but transition from one phase to another can only happen in the
order described in the above list.

An example of all of the phases Perl code can see:

BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }

INIT { print "init-time: ${^GLOBAL_PHASE}\n" }

CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }

{
package Print::Phase;

sub new {
my ($class, $time) = @_;
return bless \$time, $class;
}

sub DESTROY {
my $self = shift;
print "$$self: ${^GLOBAL_PHASE}\n";
}
}

print "run-time: ${^GLOBAL_PHASE}\n";

my $runtime = Print::Phase->new(
"lexical variables are garbage collected before END"
);

END { print "end-time: ${^GLOBAL_PHASE}\n" }

our $destruct = Print::Phase->new(
"package variables are garbage collected after END"
);

This will print out

compile-time: START
check-time: CHECK
init-time: INIT
run-time: RUN
lexical variables are garbage collected before END: RUN
end-time: END
package variables are garbage collected after END: DESTRUCT

This variable was added in Perl 5.14.0.

$^H WARNING: This variable is strictly for internal use only. Its
availability, behavior, and contents are subject to change
without notice.

This variable contains compile-time hints for the Perl
interpreter. At the end of compilation of a BLOCK the value of
this variable is restored to the value when the interpreter
started to compile the BLOCK.

When perl begins to parse any block construct that provides a
lexical scope (e.g., eval body, required file, subroutine body,
loop body, or conditional block), the existing value of $^H is
saved, but its value is left unchanged. When the compilation of
the block is completed, it regains the saved value. Between the
points where its value is saved and restored, code that executes
within BEGIN blocks is free to change the value of $^H.

This behavior provides the semantic of lexical scoping, and is
used in, for instance, the "use strict" pragma.

The contents should be an integer; different bits of it are used
for different pragmatic flags. Here's an example:

sub add_100 { $^H |= 0x100 }

sub foo {
BEGIN { add_100() }
bar->baz($boon);
}

Consider what happens during execution of the BEGIN block. At
this point the BEGIN block has already been compiled, but the
body of "foo()" is still being compiled. The new value of $^H
will therefore be visible only while the body of "foo()" is
being compiled.

Substitution of "BEGIN { add_100() }" block with:

BEGIN { require strict; strict->import('vars') }

demonstrates how "use strict 'vars'" is implemented. Here's a
conditional version of the same lexical pragma:

BEGIN {
require strict; strict->import('vars') if $condition
}

This variable was added in Perl 5.003.

%^H The "%^H" hash provides the same scoping semantic as $^H. This
makes it useful for implementation of lexically scoped pragmas.
See perlpragma.

When putting items into "%^H", in order to avoid conflicting
with other users of the hash there is a convention regarding
which keys to use. A module should use only keys that begin with
the module's name (the name of its main package) and a "/"
character. For example, a module "Foo::Bar" should use keys such
as "Foo::Bar/baz".

This variable was added in Perl 5.6.

${^OPEN}
An internal variable used by PerlIO. A string in two parts,
separated by a "\0" byte, the first part describes the input
layers, the second part describes the output layers.

This variable was added in Perl 5.8.0.

$PERLDB
$^P The internal variable for debugging support. The meanings of the
various bits are subject to change, but currently indicate:

0x01 Debug subroutine enter/exit.

0x02 Line-by-line debugging. Causes "DB::DB()" subroutine to be
called for each statement executed. Also causes saving
source code lines (like 0x400).

0x04 Switch off optimizations.

0x08 Preserve more data for future interactive inspections.

0x10 Keep info about source lines on which a subroutine is
defined.

0x20 Start with single-step on.

0x40 Use subroutine address instead of name when reporting.

0x80 Report "goto &subroutine" as well.

0x100 Provide informative "file" names for evals based on the
place they were compiled.

0x200 Provide informative names to anonymous subroutines based
on the place they were compiled.

0x400 Save source code lines into "@{"_<$filename"}".

Some bits may be relevant at compile-time only, some at run-time
only. This is a new mechanism and the details may change. See
also perldebguts.

${^TAINT}
Reflects if taint mode is on or off. 1 for on (the program was
run with -T), 0 for off, -1 when only taint warnings are enabled
(i.e. with -t or -TU).

This variable is read-only.

This variable was added in Perl 5.8.

${^UNICODE}
Reflects certain Unicode settings of Perl. See perlrun
documentation for the "-C" switch for more information about the
possible values.

This variable is set during Perl startup and is thereafter
read-only.

This variable was added in Perl 5.8.2.

${^UTF8CACHE}
This variable controls the state of the internal UTF-8 offset
caching code. 1 for on (the default), 0 for off, -1 to debug the
caching code by checking all its results against linear scans,
and panicking on any discrepancy.

This variable was added in Perl 5.8.9.

${^UTF8LOCALE}
This variable indicates whether a UTF-8 locale was detected by
perl at startup. This information is used by perl when it's in
adjust-utf8ness-to-locale mode (as when run with the "-CL"
command-line switch); see perlrun for more info on this.

This variable was added in Perl 5.8.8.

Deprecated and removed variables
Deprecating a variable announces the intent of the perl maintainers to
eventually remove the variable from the language. It may still be
available despite its status. Using a deprecated variable triggers a
warning.

Once a variable is removed, its use triggers an error telling you the
variable is unsupported.

See perldiag for details about error messages.

$OFMT
$# $# was a variable that could be used to format printed numbers.
After a deprecation cycle, its magic was removed in Perl 5.10
and using it now triggers a warning: "$# is no longer
supported".

This is not the sigil you use in front of an array name to get
the last index, like $#array. That's still how you get the last
index of an array in Perl. The two have nothing to do with each
other.

Deprecated in Perl 5.

Removed in Perl 5.10.

$* $* was a variable that you could use to enable multiline
matching. After a deprecation cycle, its magic was removed in
Perl 5.10. Using it now triggers a warning: "$* is no longer
supported". You should use the "/s" and "/m" regexp modifiers
instead.

Deprecated in Perl 5.

Removed in Perl 5.10.

$ARRAY_BASE
$[ This variable stores the index of the first element in an array,
and of the first character in a substring. The default is 0, but
you could theoretically set it to 1 to make Perl behave more
like awk (or Fortran) when subscripting and when evaluating the
index() and substr() functions.

As of release 5 of Perl, assignment to $[ is treated as a
compiler directive, and cannot influence the behavior of any
other file. (That's why you can only assign compile-time
constants to it.) Its use is highly discouraged.

Prior to Perl 5.10, assignment to $[ could be seen from outer
lexical scopes in the same file, unlike other compile-time
directives (such as strict). Using local() on it would bind its
value strictly to a lexical block. Now it is always lexically
scoped.

As of Perl 5.16, it is implemented by the arybase module. See
arybase for more details on its behaviour.

Under "use v5.16", or "no feature "array_base"", $[ no longer
has any effect, and always contains 0. Assigning 0 to it is
permitted, but any other value will produce an error.

Mnemonic: [ begins subscripts.

Deprecated in Perl 5.12.

$OLD_PERL_VERSION
$] See "$^V" for a more modern representation of the Perl version
that allows accurate string comparisons.

The version + patchlevel / 1000 of the Perl interpreter. This
variable can be used to determine whether the Perl interpreter
executing a script is in the right range of versions:

warn "No checksumming!\n" if $] < 3.019;

The floating point representation can sometimes lead to
inaccurate numeric comparisons.

See also the documentation of "use VERSION" and "require
VERSION" for a convenient way to fail if the running Perl
interpreter is too old.

Mnemonic: Is this version of perl in the right bracket?

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