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perlvar - Perl predefined variables


Predefined Names

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.

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,


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 $^OS_ERROR;
    local $/; # enable localized slurp mode
    my $content = <$fh>;
    close $fh;

But the following code is quite bad:

    open my $fh, "<", "foo" or die $^OS_ERROR;
    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 $^OS_ERROR;
        local $/;
        $content = <$fh>;
    close $fh;

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

    for (1..5){
        print "$_ ";
    sub nasty_break {
        $_ = 5;
        # do something with $_

You probably expect this code to print:

    1 2 3 4 5

but instead you get:

    5 5 5 5 5

Why? Because nasty_break() modifies $_ without localizing it first. The fix is to add local():

        local $_ = 5;

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.

The following list is ordered by scalar variables first, then the arrays, then the hashes.


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

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

    $_ =~ m/^Subject:/

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


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

  • The following functions:

    abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, 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// and s/// 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 5.9.1, 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.)


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.


Contains the subpattern from the corresponding set of capturing parentheses from the last pattern match, not counting patterns matched in nested blocks that have been exited already. (Mnemonic: like \digits.) These variables are all read-only and dynamically scoped to the current BLOCK.


The string matched by the last successful pattern match of a pattern compiled or executed with the /p modifier (not counting any matches hidden within a BLOCK or eval() enclosed by the current BLOCK). This variable is read-only and dynamically scoped to the current BLOCK.


The string preceding whatever was matched by the last successful pattern match of a pattern compiled or executed with the /p modifier (not counting any matches hidden within a BLOCK or eval() enclosed by the current BLOCK). This variable is read-only and dynamically scoped to the current BLOCK.


The string following whatever was matched by the last successful pattern match of a pattern compiled or executed with the /p modifier (not counting any matches hidden within a BLOCK or eval() enclosed by the current BLOCK). This variable is read-only and dynamically scoped to the current BLOCK.


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

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 is dynamically scoped to the current BLOCK.


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. (Mnemonic: / delimits line boundaries when quoting poetry.)

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

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

Setting $^INPUT_RECORD_SEPARATOR 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 $^INPUT_RECORD_SEPARATOR = \32768; # or \"32768", or \$var_containing_32768
    open my $fh, "<", $myfile or die $^OS_ERROR;
    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, record reads are done with the equivalent of sysread, so it's best not to mix record and non-record reads on the same file. (This is unlikely to be a problem, because any file you'd want to read in record mode is probably unusable in line mode.) Non-VMS systems do normal I/O, so it's safe to mix record and non-record reads of a file.

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


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; $^OUTPUT_AUTOFLUSH 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 perldoc on how to select the output channel. See also IO::Handle. (Mnemonic: when you want your pipes to be piping hot.)

IO::Handle->output_field_separator EXPR

The output field separator for the print operator. If defined, this value is printed between each of print's arguments. Default is undef.

IO::Handle->output_record_separator EXPR

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 $^OUTPUT_RECORD_SEPARATOR instead of adding "\n" at the end of the print. Also, it's just like $^INPUT_RECORD_SEPARATOR, but it's what you get "back" from Perl.)


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 ($^CHILD_ERROR >> 8), and $^CHILD_ERROR ^&^ 127 gives which signal, if any, the process died from, and $^CHILD_ERROR ^&^ 128 reports whether there was a core dump. (Mnemonic: similar to sh and ksh.)

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

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

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

    END {
        $^CHILD_ERROR = 1 if $^CHILD_ERROR == 255;  # die would make it 255

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

Also see "Error Indicators".


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 $^CHILD_ERROR when the pragma use vmsish 'status' is in effect.


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.


If used numerically, yields the current value of the C errno variable, or in other words, if a system or library call fails, it sets this variable. This means that the value of $^OS_ERROR is meaningful only immediately after a failure:

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

In the above meaningless stands for anything: zero, non-zero, undef. A successful system or library call does not set the variable to zero.

If used as a string, yields the corresponding system error string. You can assign a number to $^OS_ERROR to set errno if, for instance, you want "$^OS_ERROR" to return the string for error n, or you want to set the exit value for the die() operator. (Mnemonic: What just went bang?)

Also see "Error Indicators".


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

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 $^OS_ERROR. This is particularly important when $^OS_ERROR 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 $^OS_ERROR.

Caveats mentioned in the description of $^OS_ERROR generally apply to $^E, also. (Mnemonic: Extra error explanation.)

Also see "Error Indicators".


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

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

Also see "Error Indicators".


The process number of the Perl running this script. You should consider this variable read-only, although it will be altered across fork() calls.

Note for Linux users: on Linux, the C functions getpid() and getppid() return different values from different threads. In order to be portable, this behavior is not reflected by $^PID, whose value remains consistent across threads. If you want to call the underlying getpid(), you may use the CPAN module Linux::Pid.


The real uid of this process. (Mnemonic: it's the uid you came from, if you're running setuid.) You can change both the real uid and the effective uid at the same time by using POSIX::setuid(). Since changes to $^UID require a system call, check $^OS_ERROR after a change attempt to detect any possible errors.


The effective uid of this process. Example:

    $^UID = $^EUID;             # set real to effective uid
    @($^UID,$^EUID) = @($^EUID,$^UID);  # swap real and effective uid

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

(Mnemonic: it's the uid you went to, if you're running setuid.) $^UID and $^EUID can be swapped only on machines supporting setreuid().


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 $^GID must be a single number used to set the real gid. So the value given by $^GID should not be assigned back to $^GID without being forced numeric, such as by adding zero. Note that this is different to the effective gid ($^EGID) 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 $^GID require a check to $^OS_ERROR 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 $^EGID 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 $^EGID = "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 $^OS_ERROR to detect any possible errors after an attempted change.

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

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


Contains the name of the program being executed.

On some (read: 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. (Mnemonic: same as sh and ksh.)

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.


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.


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


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.


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.


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().


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() }

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 the above BEGIN 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 }

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


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.


The name of the operating system under which this copy of Perl was built, as determined during the configuration process. 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.


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.


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


Debug subroutine enter/exit.


Line-by-line debugging.


Switch off optimizations.


Preserve more data for future interactive inspections.


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


Start with single-step on.


Use subroutine address instead of name when reporting.


Report goto &subroutine as well.


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


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


Debug assertion subroutines enter/exit.

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


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.


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.


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 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 indicates whether an 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.


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. (Mnemonic: use ^V for Version Control.) 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.


The routine indicated by $^WARN_HOOK is called when a warning message is about to be printed. The warning message is passed as the first argument. The default $^WARN_HOOK calls message on the object and print it to STDERR. You can use this to save warnings in a variable, or turn warnings into fatal errors, like this:

    local $^WARN_HOOK = sub { die @_[0] };
    eval $proggie;

To save warnings:

    local $^WARN_HOOK = sub { push @warn, @_[0] };

The routine indicated by $^DIE_HOOK is called when a fatal exception is about to be thrown. The error message is passed as the first argument. The <$^DIE_HOOK> is disabled inside an eval().


The current value of the warning switch, initially true if -w was used, false otherwise, but directly modifiable. (Mnemonic: related to the -w switch.) See also warnings.


The current set of warning checks enabled by the use warnings pragma. See the documentation of warnings for more details.


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 file to configure the local Perl installation to use "sloppy" stat() by default. See perlrun for more information about site customization.


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

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.


contains the name of the current file when reading from <>.


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.


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.


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 $^INCLUDE_PATH 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 $^INCLUDE_PATH. Those hooks may be subroutine references, array references or blessed objects. See "require" in perlfunc for details.


Within a subroutine the array @_ contains the parameters passed to that subroutine. See perlsub.


The hash $^INCLUDED 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 $^INCLUDED in place of a filename. Note, however, that the hook may have set the $^INCLUDED entry by itself to provide some more specific info.


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


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.

Here are some other examples:

    %SIG{"PIPE"} = \&Plumber;   # just fine; assume current Plumber
    %SIG{"PIPE"} = *Plumber;    # somewhat esoteric
    %SIG{"PIPE"} = Plumber();   # oops, what did Plumber() return??

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.

Error Indicators

The variables $^EVAL_ERROR, $^OS_ERROR, $^E, and $^CHILD_ERROR 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:

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

After execution of this statement all 4 variables may have been set.

$^EVAL_ERROR 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 $^EVAL_ERROR is the compile error, or the argument to die (which will interpolate $^OS_ERROR and $^CHILD_ERROR). (See also Fatal, though.)

When the eval() expression above is executed, open(), <PIPE>, and close are translated to calls in the C run-time library and thence to the operating system kernel. $^OS_ERROR is set to the C library's errno if one of these calls fails.

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 $^OS_ERROR.

Finally, $^CHILD_ERROR 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 $^OS_ERROR and $^E, which are set only if error condition is detected, the variable $^CHILD_ERROR is set on each wait or pipe close, overwriting the old value. This is more like $^EVAL_ERROR, which on every eval() is always set on failure and cleared on success.

For more details, see the individual descriptions at $^EVAL_ERROR, $^OS_ERROR, $^E, and $^CHILD_ERROR.

Technical Note on 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 ::. In this case, the part before the last :: is taken to be a package qualifier; see perlmod.

Perl variable names may also be a sequence of digits or a single punctuation or a name starting with ^. 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 variable names may be alphanumeric strings that begin with a caret. 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         _

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


Due to an unfortunate accident of Perl's implementation, use English imposes a considerable performance penalty on all regular expression matches in a program, regardless of whether they occur in the scope of use English. For that reason, saying use English in libraries is strongly discouraged. See the Devel::SawAmpersand module documentation from CPAN ( ) for more information. Writing use English '-no_match_vars'; avoids the performance penalty.

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