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perlvar - Man Page

Perl predefined variables


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. A Unicode letter that is not ASCII is not considered to be a letter unless "use utf8" is in effect, and somewhat more complicated rules apply; see "Identifier parsing" in perldata for details.

Perl variable names may also be a sequence of digits, a single punctuation character, or the two-character sequence: ^ (caret or CIRCUMFLEX ACCENT) followed by any one of the characters [][A-Z^_?\]. 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.

Since Perl v5.6.0, Perl variable names may also be alphanumeric strings preceded by a caret.  These must all be written using the demarcated variable form using curly braces such as ${^Foo}; the braces are not optional.  ${^Foo} denotes the scalar variable whose name is considered to be a control-F followed by two o's. (See "Demarcated variable names using braces" in perldata for more information on this form of spelling a variable name or specifying access to an element of an array or a hash). These variables are reserved for future special uses by Perl, except for the ones that begin with ^_ (caret-underscore).  No 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.

Note that you also must use the demarcated form to access subscripts of variables of this type when interpolating, for instance to access the first element of the @{^CAPTURE} variable inside of a double quoted string you would write "${^CAPTURE[0]}" and NOT "${^CAPTURE}[0]" which would mean to reference a scalar variable named ${^CAPTURE} and not index 0 of the magic @{^CAPTURE} array which is populated by the regex engine.

Perl identifiers that begin with digits 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

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.  For more info, please see English.

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


The default input and pattern-searching space.  The following pairs are equivalent:

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

    $_ =~ /^Subject:/

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


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, fc, glob, hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf, quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only), rmdir, say, sin, split (for 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 the next value or input record when a <FH>, readline, readdir or each operation's result is tested by itself as the sole criterion of a while test.  Outside a while test, this will not happen.

$_ is a global variable.

However, between perl v5.10.0 and v5.24.0, it could be used lexically by writing my $_.  Making $_ refer to the global $_ in the same scope was then possible with our $_.  This experimental feature was removed and is now a fatal error, but you may encounter it in older code.

Mnemonic: underline is understood in certain operations.


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

See perlsub.


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.


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 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.


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 v5.14.0 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.

Wide characters are invalid in $0 values. For historical reasons, though, Perl accepts them and encodes them to UTF-8. When this happens a wide-character warning is triggered.

Mnemonic: same as sh and ksh.


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.


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.


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.


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.


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


it really means

    $foo{join($;, $x, $y, $z)}

But don't put

    @foo{$x,$y,$z}     # a slice--note the @

which means


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.


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.


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

As of v5.18.0, both keys and values stored in %ENV are stringified.

    my $foo = 1;
    $ENV{'bar'} = \$foo;
    if( ref $ENV{'bar'} ) {
        say "Pre 5.18.0 Behaviour";
    } else {
        say "Post 5.18.0 Behaviour";

Previously, only child processes received stringified values:

    my $foo = 1;
    $ENV{'bar'} = \$foo;

    # Always printed 'non ref'
    system($^X, '-e',
           q/print ( ref $ENV{'bar'}  ? 'ref' : 'non ref' ) /);

This happens because you can't really share arbitrary data structures with foreign processes.


The revision, version, and subversion of the Perl interpreter, represented as a decimal of the form 5.XXXYYY, where XXX is the version / 1e3 and YYY is the subversion / 1e6.  For example, Perl v5.10.1 would be "5.010001".

This variable can be used to determine whether the Perl interpreter executing a script is in the right range of versions:

    warn "No PerlIO!\n" if "$]" < 5.008;

When comparing $], numeric comparison operators should be used, but the variable should be stringified first to avoid issues where its original numeric value is inaccurate.

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

See "$^V" for a representation of the Perl version as a version object, which allows more flexible string comparisons.

The main advantage of $] over $^V is that it works the same on any version of Perl.  The disadvantages are that it can't easily be compared to versions in other formats (e.g. literal v-strings, "v1.2.3" or version objects) and numeric comparisons are subject to the binary floating point representation; it's good for numeric literal version checks and bad for comparing to a variable that hasn't been sanity-checked.

The $OLD_PERL_VERSION form was added in Perl v5.20.0 for historical reasons but its use is discouraged. (If your reason to use $] is to run code on old perls then referring to it as $OLD_PERL_VERSION would be self-defeating.)

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


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().


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'.


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. Prior to Perl 5.26, . -which represents the current directory, was included in @INC; it has been removed. This change in behavior is documented in PERL_USE_UNSAFE_INC and it is not recommended that . be re-added to @INC. If you need to modify @INC at runtime, you should use the use lib pragma to get the machine-dependent library properly loaded as well:

    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.


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.


As of 5.37.7 when an @INC hook is executed the index of the @INC array that holds the hook will be localized into the $INC variable. When the hook returns the integer successor of its value will be used to determine the next index in @INC that will be checked, thus if it is set to -1 (or undef) the traversal over the @INC array will be restarted from its beginning.

Normally traversal through the @INC array is from beginning to end (0 .. $#INC), and if the @INC array is modified by the hook the iterator may be left in a state where newly added entries are skipped. Changing this value allows an @INC hook to rewrite the @INC array and tell Perl where to continue afterwards. See "require" in perlfunc for details on @INC hooks.


The current value of the inplace-edit extension.  Use undef to disable inplace editing.

Mnemonic: value of -i switch.


Each package contains a special array called @ISA which contains a list of that class's parent classes, if any. This array is simply a list of scalars, each of which is a string that corresponds to a package name. The array is examined when Perl does method resolution, which is covered in perlobj.

To load packages while adding them to @ISA, see the parent pragma. The discouraged base pragma does this as well, but should not be used except when compatibility with the discouraged fields pragma is required.


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.


This variable determines the maximum number eval EXPR/BEGIN or require/BEGIN block nesting that is allowed. This means it also controls the maximum nesting of use statements as well.

The default of 1000 should be sufficiently large for normal working purposes, and if you must raise it then you should be conservative with your choice or you may encounter segfaults from exhaustion of the C stack. It seems unlikely that real code has a use depth above 1000, but we have left this configurable just in case.

When set to 0 then BEGIN blocks inside of eval EXPR or require EXPR are forbidden entirely and will trigger an exception which will terminate the compilation and in the case of require will throw an exception, or in the case of eval return the error in $@ as usual.

Consider the code

 perl -le'sub f { eval "BEGIN { f() }"; } f()'

each invocation of f() will consume considerable C stack, and this variable is used to cause code like this to die instead of exhausting the C stack and triggering a segfault. Needless to say code like this is unusual, it is unlikely you will actually need to raise the setting. However it may be useful to set it to 0 for a limited time period to prevent BEGIN{} blocks from being executed during an eval EXPR.

Note that setting this to 1 would NOT affect code like this:

    BEGIN { $n += 1; BEGIN { $n += 2; BEGIN { $n += 4 } } }

The reason is that BEGIN blocks are executed immediately after they are completed, thus the innermost will execute before the ones which contain it have even finished compiling, and the depth will not go above 1. In fact the above code is equivalent to

    BEGIN { $n+=4 }
    BEGIN { $n+=2 }
    BEGIN { $n+=1 }

which makes it obvious why a ${^MAX_EVAL_BEGIN_DEPTH} of 1 would not block this code.

Only BEGIN's executed inside of an eval or require (possibly via use) are affected.


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.


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";

    $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.  Using an empty string or undef as the value has the same effect as 'DEFAULT'.

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.

Using a string that doesn't correspond to any existing function or a glob that doesn't contain a code slot is equivalent to 'IGNORE', but a warning is emitted when the handler is being called (the warning is not emitted for the internal hooks described below).

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 v5.8.0 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__, its effect is the same as using 'DEFAULT'.  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__.

The $SIG{__DIE__} hook is called even inside an eval(). It was never intended to happen this way, but an implementation glitch made this possible. This used to be deprecated, as it allowed strange action at a distance like rewriting a pending exception in $@. Plans to rectify this have been scrapped, as users found that rewriting a pending exception is actually a useful feature, and not a bug.

The $SIG{__DIE__} doesn't support 'IGNORE'; it has the same effect as 'DEFAULT'.

__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.


This hash contains coderefs which are called when various perl keywords which are hard or impossible to wrap are called. The keys of this hash are named after the keyword that is being hooked, followed by two underbars and then a phase term; either "before" or "after".

Perl will throw an error if you attempt modify a key which is not documented to exist, or if you attempt to store anything other than a code reference or undef in the hash.  If you wish to use an object to implement a hook you can use currying to embed the object into an anonymous code reference.

Currently there is only one keyword which can be hooked, require, but it is expected that in future releases there will be additional keywords with hook support.


The routine indicated by ${^HOOK}{require__before} is called by require before it checks %INC, looks up @INC, calls INC hooks, or compiles any code.  It is called with a single argument, the filename for the item being required (package names are converted to paths).  It may alter this filename to change what file is loaded.  If the hook dies during execution then it will block the require from executing.

In order to make it easy to perform an action with shared state both before and after the require keyword was executed the require__before hook may return a "post-action" coderef which will in turn be executed when the require completes.  This coderef will be executed regardless as to whether the require completed succesfully or threw an exception.  It will be called with the filename that was required.  You can check %INC to determine if the require was successful.  Any other return from the require__before hook will be silently ignored.

require__before hooks are called in FIFO order, and if the hook returns a code reference those code references will be called in FILO order.  In other words if A requires B requires C, then require__before will be called first for A, then B and then C, and the post-action code reference will executed first for C, then B and then finally A.

Well behaved code should ensure that when setting up a require__before hook that any prior installed hook will be called, and that their return value, if a code reference, will be called as well.  See "require" in perlfunc for an example implementation.


The routine indicated by ${^HOOK}{require__after} is called by require after the require completes.  It is called with a single argument, the filename for the item being required (package names are converted to paths).  It is executed when the require completes, either via exception or via completion of the require statement, and you can check %INC to determine if the require was successful.

The require__after hook is called for each required file in FILO order. In other words if A requires B requires C, then require__after will be called first for C, then B and then A.


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.


The revision, version, and subversion of the Perl interpreter, represented as a version object.

This variable first appeared in perl v5.6.0; earlier versions of perl will see an undefined value.  Before perl v5.10.0 $^V was represented as a v-string rather than a version object.

$^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

While version objects overload stringification, to portably convert $^V into its string representation, use sprintf()'s "%vd" conversion, which works for both v-strings or version objects:

    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 a decimal representation of the Perl version.

The main advantage of $^V over $] is that, for Perl v5.10.0 or later, it overloads operators, allowing easy comparison against other version representations (e.g. decimal, literal v-string, "v1.2.3", or objects).  The disadvantage is that prior to v5.10.0, it was only a literal v-string, which can't be easily printed or compared, whereas the behavior of $] is unchanged on all versions of Perl.

Mnemonic: use ^V for a version object.


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;

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 a 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 an 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 $?.


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, and WSTOPSIG 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 v5.10.0.


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.


Current state of the interpreter.

    $^S         State
    ---------   -------------------------------------
    undef       Parsing module, eval, or main program
    true (1)    Executing an eval or try block
    false (0)   Otherwise

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

The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because the undef value does not indicate whether exceptions are being caught, since compilation of the main program does not catch exceptions.

This variable was added in Perl 5.004.


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.


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.

Each time a statement completes being compiled, the current value of ${^WARNING_BITS} is stored with that statement, and can later be retrieved via (caller($level))[9].

This variable was added in Perl v5.6.0.


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 and may set errno to a non-zero value on success.  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.

Perl itself may set errno to a non-zero on failure even if no system call is performed.

Mnemonic: What just went bang?


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).  The specific true value is not guaranteed, but in the past has generally been the numeric value of $!.  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.


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.


The Perl error from the last eval operator, i.e. the last exception that was caught.  For eval BLOCK, this is either a runtime error message or the string or reference die was called with.  The eval STRING form also catches syntax errors and other compile time exceptions.

If no error occurs, eval sets $@ to the empty string.

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 error "at"?

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.


$# was a variable that could be used to format printed numbers. After a deprecation cycle, its magic was removed in Perl v5.10.0 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 v5.10.0.


$* was a variable that you could use to enable multiline matching. After a deprecation cycle, its magic was removed in Perl v5.10.0. 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 v5.10.0.


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 v5.10.0, 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 v5.16.0, it is implemented by the arybase module.

As of Perl v5.30.0, or 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 v5.12.0.


This variable is no longer supported.

It used to hold the object reference to the Encode object that was used to convert the source code to Unicode.

Its purpose was to allow your non-ASCII Perl scripts not to have to be written in UTF-8; this was useful before editors that worked on UTF-8 encoded text were common, but that was long ago.  It caused problems, such as affecting the operation of other modules that weren't expecting it, causing general mayhem.

If you need something like this functionality, it is recommended that use you a simple source filter, such as Filter::Encoding.

If you are coming here because code of yours is being adversely affected by someone's use of this variable, you can usually work around it by doing this:

 local ${^ENCODING};

near the beginning of the functions that are getting broken.  This undefines the variable during the scope of execution of the including function.

This variable was added in Perl 5.8.2 and removed in 5.26.0. Setting it to anything other than undef was made fatal in Perl 5.28.0.


This variable no longer has any function.

This variable was added in Perl v5.10.0 and removed in Perl v5.34.0.


2024-06-12 perl v5.40.0 Perl Programmers Reference Guide