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NAME    [Toc]    [Back]

       perlvar - Perl predefined variables

DESCRIPTION    [Toc]    [Back]

       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. In general,
 it's best to use the

           use English '-no_match_vars';

       invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH,
 as it avoids a certain performance hit with the use
       of regular expressions. See English.

       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 builtin

       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..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 (defined($_ = <>)) {...}

                   $_ =~ /^Subject:/

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


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

               *  Various unary functions, including functions
                  like ord() and int(), as well as the all file
                  tests ("-f", "-d") except for "-t", which
                  defaults to STDIN.

               *  Various list functions like print() and

               *  The pattern matching operations "m//", "s///",
                  and "tr///" 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 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.

               (Mnemonic: underline is understood in certain

       $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.

               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 igits.)  These variables are all read-only
               and dynamically scoped to the current BLOCK.

       $&      The string matched by the last successful pattern
               match (not counting any matches hidden within a
               BLOCK or eval() enclosed by the current BLOCK).
               (Mnemonic: like & in some editors.)  This variable
               is read-only and dynamically scoped to the current

               The use of this variable anywhere in a program
               imposes a considerable performance penalty on all
               regular expression matches.  See "BUGS".

       $`      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).  (Mnemonic: "`" often precedes
               a quoted string.)  This variable is read-only.

               The use of this variable anywhere in a program
               imposes a considerable performance penalty on all
               regular expression matches.  See "BUGS".

       $'      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).  (Mnemonic: "'" often follows
 a quoted string.)  Example:
                   local $_ = 'abcdefghi';
                   print "$`:$&:$'0;         # prints abc:def:ghi

               This variable is read-only and dynamically scoped
               to the current BLOCK.

               The use of this variable anywhere in a program
               imposes a considerable performance penalty on all
               regular expression matches.  See "BUGS".

       $+      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 = $+);

               (Mnemonic: be positive and forward looking.)  This
               variable is read-only and dynamically scoped to
               the current BLOCK.

       $^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.
  (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

       @+      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 nth element of this array holds the
               offset of the nth 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.

       $*      Set to a non-zero integer value to do multi-line
               matching within a string, 0 (or undefined) to tell
               Perl that it can assume that strings contain a
               single line, for the purpose of optimizing pattern
               matches.  Pattern matches on strings containing
               multiple newlines can produce confusing results
               when $* is 0 or undefined. Default is undefined.
               (Mnemonic: * matches multiple things.) This variable
 influences the interpretation of only "^" and
               "$". A literal newline can be searched for even
               when "$* == 0".

               Use of $* is deprecated in modern Perl, supplanted
               by the "/s" and "/m" modifiers on pattern matching.

               Assigning a non-numerical value to $* triggers a
               warning (and makes $* act if "$* == 0"), while
               assigning a numerical value to $* makes that an
               implicit "int" is applied on the value.

       $.      Current line number for the last filehandle

               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

               $. 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.)

       $/      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 "0 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 "0 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 $/;           # enable "slurp" mode
                   local $_ = <FH>;    # whole file now here
                   s/]+/ /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
                   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 recordoriented
 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.

               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

               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; $| 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.  (Mnemonic: when you want your pipes to
               be piping hot.)

       IO::Handle->output_field_separator EXPR
       $,      The output field separator for the print operator.
               Ordinarily the print operator simply prints out
               its arguments without further adornment.  To get
               behavior more like awk, set this variable as you
               would set awk's OFS variable to specify what is
               printed between fields.  (Mnemonic: what is
               printed when there is a "," in your print statement.)

       IO::Handle->output_record_separator EXPR
       $      The output record separator for the print operator.
  Ordinarily the print operator simply prints
               out its arguments as is, with no trailing newline
               or other end-of-record string added.  To get
               behavior more like awk, set this variable as you
               would set awk's ORS variable to specify what is
               printed at the end of the print.  (Mnemonic: you
               set "$
               print.  Also, it's just like $/, but it's what you
               get "back" from Perl.)

       $"      This is like $, except that it applies to array
               and slice values interpolated into a double-quoted
               string (or similar interpreted string).  Default
               is a space.  (Mnemonic: obvious, I think.)

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


               it really means

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

               But don't put

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

               which means


               Default is " 34", the same as SUBSEP in awk.  If
               your keys contain binary data there might not be
               any safe value for $;.  (Mnemonic: comma (the syntactic
 subscript separator) is a semi-semicolon.
               Yeah, I know, it's pretty lame, but $, is already
               taken for something more important.)

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

       $#      The output format for printed numbers.  This variable
 is a half-hearted attempt to emulate awk's
               OFMT variable.  There are times, however, when awk
               and Perl have differing notions of what counts as
               numeric.  The initial value is "%.ng", where n is
               the value of the macro DBL_DIG from your system's
               float.h.  This is different from awk's default
               OFMT setting of "%.6g", so you need to set $#
               explicitly to get awk's value.  (Mnemonic: # is
               the number sign.)

               Use of $# is deprecated.

       $%      The current page number of the currently selected
               output channel.  Used with formats.  (Mnemonic: %
               is page number in nroff.)

       $=      The current page length (printable lines) of the
               currently selected output channel.  Default is 60.
               Used with formats.  (Mnemonic: = has horizontal

       $-      The number of lines left on the page of the currently
 selected output channel.  Used with formats.
   (Mnemonic: lines_on_page - lines_printed.)

       @-      $-[0] is the offset of the start of the last successful
 match.  "$-["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.  "$-[0]" is the offset into
               the string of the beginning of the entire match.
               The nth element of this array holds the offset of
               the nth submatch, so "$-[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] -
               $' is the same as "substr($var, $+[0])"
               $1 is the same as "substr($var, $-[1], $+[1] -
               $2 is the same as "substr($var, $-[2], $+[2] -
               $3 is the same as "substr $var, $-[3], $+[3] -
       $~      The name of the current report format for the currently
 selected output channel.  Default is the
               name of the filehandle.  (Mnemonic: brother to

       $^      The name of the current top-of-page format for the
               currently selected output channel.  Default is the
               name of the filehandle with _TOP appended.
               (Mnemonic: points to top of page.)

       IO::Handle->format_line_break_characters EXPR
       $:      The current set of characters after which a string
               may be broken to fill continuation fields (starting
 with ^) in a format.  Default is " 0, to
               break on whitespace or hyphens.  (Mnemonic: a
               "colon" in poetry is a part of a line.)

       IO::Handle->format_formfeed EXPR
       $^L     What formats output as a form feed.  Default is

       $^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 "form-
               line()" in perlfunc.

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

               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

               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.

               Also see "Error Indicators".

               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.  See encoding for more details.

       $!      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 $! is meaningful
 only immediately after a failure:

                   if (open(FH, $filename)) {
                       # Here $! is meaningless.
                   } else {
                       # ONLY here is $! meaningful.
                       # Already here $! might be meaningless.
                   #  Since  here we might have either success or
                   # here $! 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 $!
               to set errno if, for instance, you want "$!" 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".
       %!      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 above for the validity of $!.

       $^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.  (Mnemonic: Extra error

               Also see "Error Indicators".

       $@      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).  (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.  (Mnemonic: same as shells.)

               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 $$, 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().

       $>      The effective uid of this process.  Example:

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

               You can change both the effective uid and the real
               uid at the same time by using POSIX::setuid().

               (Mnemonic: it's the uid you went to, if you're
               running setuid.)  $< and $> 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 $( 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

               You can change both the real gid and the effective
               gid at the same time by using POSIX::setgid().

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

       $)      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).

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

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

       $0      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

               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 the view of $0 the other threads have
               will not change since they have their own copies
               of it.

       $[      The index of the first element in an array, and of
               the first character in a substring.  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 sub-
               str() functions.  (Mnemonic: [ begins subscripts.)

               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.

               Note that, unlike other compile-time directives
               (such as strict), assignment to $[ can be seen
               from outer lexical scopes in the same file.  However,
 you can use local() on it to strictly bound
               its value to a lexical block.

       $]      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.  (Mnemonic: Is
               this version of perl in the right bracket?)  Example:

                   warn "No checksumming!0 if $] < 3.019;

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

               When testing the variable, to steer clear of
               floating point inaccuracies you might want to prefer
 the inequality tests "<" and ">" to the tests
               containing equivalence: "<=", "==", and ">=".

               The floating point representation can sometimes
               lead to inaccurate numeric comparisons.  See $^V
               for a more modern representation of the Perl version
 that allows accurate string comparisons.

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

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

       $^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().

       $^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

               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 }

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

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

       $^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 enable this option.
               To discourage casual use of this advanced feature,
               there is no English long name for this variable.

       $^O     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 " " byte, the first
               part describes the input layers, the second part
               describes the output layers.

       $^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.

               0x04  Switch off optimizations.

               0x08  Preserve more data for future interactive

               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

               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 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.

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

       $^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.

       $^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.

               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

               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

       $^V     The revision, version, and subversion of the Perl
               interpreter, represented as a string composed of
               characters with those ordinals.  Thus in Perl
               v5.6.0 it equals "chr(5) . chr(6) . chr(0)" and
               will return true for "$^V eq v5.6.0".  Note that
               the characters in this string value can potentially
 be in Unicode range.

               This 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 "No

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

                   printf  "version is v%vd0, $^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.

       $^W     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.

       $^X     The name used to execute the current copy of Perl,
               from C's "argv[0]".

               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

               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
                 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;}

       ARGV    The special filehandle that iterates over commandline
 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 "RGV" 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.

       $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.

       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.

       @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.

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

       %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
               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

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

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

                   sub handler {       # 1st argument  is  signal
                       my($sig) = @_;
                       print "Caught a SIG$sig--shutting down0;

                   $SIG{'INT'}  = handler;
                   $SIG{'QUIT'} = handler;
                   $SIG{'INT'}   = 'DEFAULT';   # restore default
                   $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

               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;

               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", 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

                   require Carp if defined $^S;
                   Carp::confess("Something  wrong")  if  defined
                   die  "Something wrong, but could not load Carp
to give backtrace...
                        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.

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

       Error Indicators    [Toc]    [Back]

       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

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

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

       $@ 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.)

       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.
  $! 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 $!.

       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

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