PERLVAR(1)       Perl Programmers Reference Guide      PERLVAR(1)

NNAAMMEE
       perlvar - Perl predefined variables

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       PPrreeddeeffiinneedd NNaammeess

       The following names have special meaning to Perl.  Most
       punctuation names have reasonable mnemonics, or analogues
       in one of the shells.  Nevertheless, if you wish to use
       long variable names, you just need to say

           use English;

       at the top of your program.  This will alias all the short
       names to the long names in the current package.  Some even
       have medium names, generally borrowed from aawwkk.

       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
       (http://www.perl.com/CPAN/modules/by-module/Devel/Devel-
       SawAmpersand-0.10.readme) for more information.

       To go a step further, those variables that depend on the
       currently selected filehandle may instead (and preferably)
       be set by calling an object method on the FileHandle
       object.  (Summary lines below for this contain the word
       HANDLE.)  First you must say

           use FileHandle;

       after which you may use either

           method HANDLE EXPR

       or more safely,

           HANDLE->method(EXPR)

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

       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.

       The following list is ordered by scalar variables first,
       then the arrays, then the hashes (except $^M was added in
       the wrong place).  This is somewhat obscured by the fact
       that %ENV and %SIG are listed as $ENV{expr} and
       $SIG{expr}.

       $ARG

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

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

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

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

                   chop
                   chop($_)

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

       o          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.

       o          Various list functions like print() and
                  unlink().

       o          The pattern matching operations m//, s///, and
                  tr/// when used without an =~ operator.

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

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

       o          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.  Note that
                  outside of a while test, this will not happen.

                  (Mnemonic: underline is understood in certain
                  operations.)

       $<digits>
               Contains the subpattern from the corresponding set
               of parentheses in the last pattern matched, not
               counting patterns matched in nested blocks that
               have been exited already.  (Mnemonic: like
               \digits.)  These variables are all read-only.

       $MATCH

       $&      The string matched by the last successful pattern
               match (not counting any matches hidden within a
               BLOCK or eval() enclosed by the current BLOCK).
               (Mnemonic: like & in some editors.)  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 the
               Devel::SawAmpersand module from CPAN for more
               information.

       $PREMATCH

       $`      The string preceding whatever was matched by the
               last successful pattern match (not counting any
               matches hidden within a BLOCK or eval enclosed by
               the current BLOCK).  (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 the
               Devel::SawAmpersand module from CPAN for more
               information.

       $POSTMATCH

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

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

               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 the
               Devel::SawAmpersand module from CPAN for more
               information.

       $LAST_PAREN_MATCH

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

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

               (Mnemonic: be positive and forward looking.)  This
               variable is read-only.

       $MULTILINE_MATCHING

       $*      Set to 1 to do multi-line matching within a
               string, 0 to tell Perl that it can assume that
               strings contain a single line, for the purpose of
               optimizing pattern matches.  Pattern matches on
               strings containing multiple newlines can produce
               confusing results when "$*" is 0.  Default is 0.
               (Mnemonic: * matches multiple things.)  Note that
               this variable influences the interpretation of
               only "^" and "$".  A literal newline can be
               searched for even when $* == 0.

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

       input_line_number HANDLE EXPR

       $INPUT_LINE_NUMBER

       $NR

       $.      The current input line number for the last file
               handle from which you read (or performed a seek or
               tell on).  The value may be different from the
               actual physical line number in the file, depending
               on what notion of "line" is in effect--see the
               section on $/ on how to affect that.  An explicit
               close on a filehandle resets the line number.
               Because "<>" never does an explicit close, line
               numbers increase across ARGV files (but see
               examples under eof()).  Localizing $. has the
               effect of also localizing Perl's notion of "the
               last read filehandle".  (Mnemonic: many programs
               use "." to mean the current line number.)

       input_record_separator HANDLE EXPR

       $INPUT_RECORD_SEPARATOR

       $RS

       $/      The input record separator, newline by default.
               This is used to influence Perl's idea of what a
               "line" is.  Works like aawwkk's RS variable,
               including treating empty lines as delimiters if
               set to the null string.  (Note: An empty line
               cannot contain any spaces or tabs.)  You may set
               it to a multi-character string to match a multi-
               character delimiter, or to undef to read to end of
               file.  Note that setting it to "\n\n" means
               something slightly different than setting it to
               "", if the file contains consecutive empty lines.
               Setting it to "" will treat two or more
               consecutive empty lines as a single empty line.
               Setting it to "\n\n" will blindly assume that the
               next input character belongs to the next
               paragraph, even if it's a newline.  (Mnemonic: /
               is used to delimit line boundaries when quoting
               poetry.)

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

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

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

                   $/ = \32768; # or \"32768", or \$var_containing_32768
                   open(FILE, $myfile);
                   $_ = <FILE>;

               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.

               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 likely
               not a problem, as any file you'd want to read in
               record mode is probably usable in line mode) Non-
               VMS systems perform normal I/O, so it's safe to
               mix record and non-record reads of a file.

               Also see the section on $..

       autoflush HANDLE EXPR

       $OUTPUT_AUTOFLUSH

       $|      If set to nonzero, forces a flush right away and
               after every write or print on the currently
               selected output channel.  Default is 0 (regardless
               of whether the channel is actually buffered by the
               system or not; $| tells you only whether you've
               asked Perl explicitly to flush after each write).
               Note that 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, such
               as when you are running a Perl script under rsh
               and want to see the output as it's happening.
               This has no effect on input buffering.  (Mnemonic:
               when you want your pipes to be piping hot.)

       output_field_separator HANDLE EXPR

       $OUTPUT_FIELD_SEPARATOR

       $OFS

       $,      The output field separator for the print operator.
               Ordinarily the print operator simply prints out
               the comma-separated fields you specify.  To get
               behavior more like aawwkk, set this variable as you
               would set aawwkk's OFS variable to specify what is
               printed between fields.  (Mnemonic: what is
               printed when there is a , in your print
               statement.)

       output_record_separator HANDLE EXPR

       $OUTPUT_RECORD_SEPARATOR

       $ORS

       $\      The output record separator for the print
               operator.  Ordinarily the print operator simply
               prints out the comma-separated fields you specify,
               with no trailing newline or record separator
               assumed.  To get behavior more like aawwkk, set this
               variable as you would set aawwkk's ORS variable to
               specify what is printed at the end of the print.
               (Mnemonic: you set "$\" instead of adding \n at
               the end of the print.  Also, it's just like $/,
               but it's what you get "back" from Perl.)

       $LIST_SEPARATOR

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

       $SUBSCRIPT_SEPARATOR

       $SUBSEP

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

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

               it really means

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

               But don't put

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

               which means

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

               Default is "\034", the same as SUBSEP in aawwkk.
               Note that 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.

       $OFMT

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

               Use of "$#" is deprecated.

       format_page_number HANDLE EXPR

       $FORMAT_PAGE_NUMBER

       $%      The current page number of the currently selected
               output channel.  (Mnemonic: % is page number in
               nnrrooffff.)

       format_lines_per_page HANDLE EXPR

       $FORMAT_LINES_PER_PAGE

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

       format_lines_left HANDLE EXPR

       $FORMAT_LINES_LEFT

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

       format_name HANDLE EXPR

       $FORMAT_NAME

       $~      The name of the current report format for the
               currently selected output channel.  Default is
               name of the filehandle.  (Mnemonic: brother to
               "$^".)

       format_top_name HANDLE EXPR

       $FORMAT_TOP_NAME

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

       format_line_break_characters HANDLE EXPR

       $FORMAT_LINE_BREAK_CHARACTERS

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

       format_formfeed HANDLE EXPR

       $FORMAT_FORMFEED

       $^L     What formats output to perform a form feed.
               Default is \f.

       $ACCUMULATOR

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

       $CHILD_ERROR

       $?      The status returned by the last pipe close,
               backtick (``) command, or system() operator.  Note
               that this is the status word returned by the
               wait() system call (or else is made up to look
               like it).  Thus, the exit value of the subprocess
               is actually ($? >> 8), and $? & 127 gives which
               signal, if any, the process died from, and $? &
               128 reports whether there was a core dump.
               (Mnemonic: similar to sshh and kksshh.)

               Additionally, if the h_errno variable is supported
               in C, its value is returned via $? if any of the
               gethost*() functions fail.

               Note that 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 the script.

               Under VMS, the pragma use vmsish 'status' makes $?
               reflect the actual VMS exit status, instead of the
               default emulation of POSIX status.

               Also see the section on Error Indicators.

       $OS_ERROR

       $ERRNO

       $!      If used in a numeric context, yields the current
               value of errno, with all the usual caveats.  (This
               means that you shouldn't depend on the value of $!
               to be anything in particular unless you've gotten
               a specific error return indicating a system
               error.)  If used in a string context, yields the
               corresponding system error string.  You can assign
               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 the section on Error Indicators.

       $EXTENDED_OS_ERROR

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

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

               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 explanation.)

               Also see the section on Error Indicators.

       $EVAL_ERROR

       $@      The Perl syntax error message from the last eval()
               command.  If null, 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"?)

               Note that 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 the section on Error Indicators.

       $PROCESS_ID

       $PID

       $$      The process number of the Perl running this
               script.  (Mnemonic: same as shells.)

       $REAL_USER_ID

       $UID

       $<      The real uid of this process.  (Mnemonic: it's the
               uid you came FROM, if you're running setuid.)

       $EFFECTIVE_USER_ID

       $EUID

       $>      The effective uid of this process.  Example:

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

               (Mnemonic: it's the uid you went TO, if you're
               running setuid.)  Note: "$<" and "$>" can be
               swapped only on machines supporting setreuid().

       $REAL_GROUP_ID

       $GID

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

               However, a value assigned to "$(" must be a single
               number used to set the real gid.  So the value
               given by "$(" should not be assigned back to "$("
               without being forced numeric, such as by adding
               zero.

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

       $EFFECTIVE_GROUP_ID

       $EGID

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

               Similarly, a value assigned to "$)" must also be a
               space-separated list of numbers.  The first number
               is used to set 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" .

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

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

       $PROGRAM_NAME

       $0      Contains the name of the file containing the Perl
               script being executed.  On some operating systems
               assigning to "$0" modifies the argument area that
               the ps(1) program sees.  This 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 sshh and kksshh.)

       $[      The index of the first element in an array, and of
               the first character in a substring.  Default is 0,
               but you could set it to 1 to make Perl behave more
               like aawwkk (or Fortran) when subscripting and when
               evaluating the index() and substr() functions.
               (Mnemonic: [ begins subscripts.)

               As of Perl 5, assignment to "$[" is treated as a
               compiler directive, and cannot influence the
               behavior of any other file.  Its use is
               discouraged.

       $PERL_VERSION

       $]      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!\n" if $] < 3.019;

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

       $COMPILING

       $^C     The current value of the flag associated with the
               --cc switch. Mainly of use with --MMOO==...... to allow
               code to alter its behaviour when being compiled.
               (For example to automatically AUTOLOADing at
               compile time rather than normal deferred loading.)
               Setting $^C = 1 is similar to calling B::minus_c.

       $DEBUGGING

       $^D     The current value of the debugging flags.
               (Mnemonic: value of --DD switch.)

       $SYSTEM_FD_MAX

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

       $^H     The current set of syntax checks enabled by use
               strict and other block scoped compiler hints.  See
               the documentation of strict for more details.

       $INPLACE_EDIT

       $^I     The current value of the inplace-edit extension.
               Use undef to disable inplace editing.  (Mnemonic:
               value of --ii switch.)

       $^M     By default, running out of memory it is not
               trappable.  However, if compiled for this, Perl
               may use the contents of $^M as an emergency pool
               after die()ing with this message.  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 when in
               emergency.  See the INSTALL file for information
               on how to enable this option.  As a disincentive
               to casual use of this advanced feature, there is
               no the English manpage long name for this
               variable.

       $OSNAME

       $^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'}.

       $PERLDB

       $^P     The internal variable for debugging support.
               Different bits mean the following (subject to
               change):

       0x01          Debug subroutine enter/exit.

       0x02          Line-by-line debugging.

       0x04          Switch off optimizations.

       0x08          Preserve more data for future interactive
                     inspections.

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

       0x20          Start with single-step on.

                     Note that 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
               the section on (?{ code }) in the perlre manpage
               regular expression assertion.  (Excluding those
               used as switches.)  May be written to.

       $^S     Current state of the interpreter.  Undefined if
               parsing of the current module/eval is not finished
               (may happen in $SIG{__DIE__} and $SIG{__WARN__}
               handlers).  True if inside an eval, otherwise
               false.

       $BASETIME

       $^T     The time at which the script began running, in
               seconds since the epoch (beginning of 1970).  The
               values returned by the --MM, --AA, and --CC filetests
               are based on this value.

       $WARNING

       $^W     The current value of the warning switch, either
               TRUE or FALSE.  (Mnemonic: related to the --ww
               switch.)

       $EXECUTABLE_NAME

       $^X     The name that the Perl binary itself was executed
               as, from C's argv[0].

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

       @ARGV   The array @ARGV contains the command line
               arguments intended for the script.  Note that
               $#ARGV is the generally number of arguments minus
               one, because $ARGV[0] is the first argument, NOT
               the command name.  See "$0" for the command name.

       @INC    The array @INC contains the list of places to look
               for Perl scripts to be evaluated by the do EXPR,
               require, or use constructs.  It initially consists
               of the arguments to any --II command line switches,
               followed by the default Perl library, probably
               /usr/local/lib/perl, followed by ".", to represent
               the current directory.  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;

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

       %INC    The hash %INC contains entries for each filename
               that has been included via do or require.  The key
               is the filename you specified, and the value is
               the location of the file actually found.  The
               require command uses this array to determine
               whether a given file has already been included.

       %ENV

       $ENV{expr}
               The hash %ENV contains your current environment.
               Setting a value in ENV changes the environment for
               child processes.

       %SIG

       $SIG{expr}
               The hash %SIG is used to set signal handlers for
               various signals.  Example:

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

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

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

               The %SIG array contains values for only the
               signals actually set within the Perl script.  Here
               are some other examples:

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

               The one marked scary is problematic because it's a
               bareword, which means sometimes it's a string
               representing the function, and sometimes it's
               going to call the subroutine call right then and
               there!  Best to be sure and quote it or take a
               reference to it.  *Plumber works too.  See the
               perlsub manpage.

               If your system has the sigaction() function then
               signal handlers are installed using it.  This
               means you get reliable signal handling.  If your
               system has the SA_RESTART flag it is used when
               signals handlers are installed.  This means that
               system calls for which it is supported continue
               rather than returning when a signal arrives.  If
               you want your system calls to be interrupted by
               signal delivery then do something like this:

                   use POSIX ':signal_h';

                   my $alarm = 0;
                   sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
                       or die "Error setting SIGALRM handler: $!\n";

               See the POSIX manpage.

               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__.

               Note that the $SIG{__DIE__} hook is called even
               inside eval()ed blocks/strings.  See the die entry
               in the perlfunc manpage and the section on $^S in
               the perlvar manpage for how to circumvent this.

               Note that __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 calls which result/may-result in
               parsing Perl should be used with extreme caution,
               like this:

                   require Carp if defined $^S;
                   Carp::confess("Something wrong") if defined &Carp::confess;
                   die "Something wrong, but could not load Carp to give backtrace...
                        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 the die entry in the perlfunc manpage, the
               warn entry in the perlfunc manpage and the eval
               entry in the perlfunc manpage for additional info.

       EErrrroorr IInnddiiccaattoorrss

       The variables the section on $@, the section on $!, the
       section on $^E, and the section on $? contain information
       about different types of error conditions that may appear
       during execution of Perl script.  The variables are shown
       ordered by the "distance" between the subsystem which
       reported the error and the Perl process, and 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:

          eval '
                open PIPE, "/cdrom/install |";
                @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 (either implicitly, say, if open was imported from
       module the Fatal manpage, or the die after close was
       triggered).  In these cases the value of $@ is the compile
       error, or Fatal error (which will interpolate $!!), or the
       argument to die (which will interpolate $! and $?!).

       When the above expression is executed, open(), <PIPE>, and
       close are translated to C run-time library calls.  $! is
       set if one of these calls fails.  The value is a symbolic
       indicator chosen by the C run-time library, say No such
       file or directory.

       On some systems the above C library calls are further
       translated to calls to the kernel.  The kernel may have
       set more verbose error indicator that one of the handful
       of standard C errors.  In such cases $^E contains this
       verbose error indicator, which may be, say, CDROM tray not
       closed.  On systems where C library calls are identical to
       system calls $^E is a duplicate of $!.

       Finally, $? may be set to non-0 value if the external
       program /cdrom/install fails.  Upper bits of the
       particular value may reflect specific error conditions
       encountered by this program (this is program-dependent),
       lower-bits reflect mode of failure (segfault, completion,
       etc.).  Note that in contrast to $@, $!, and $^E, which
       are set only if error condition is detected, the variable
       $? is set on each wait or pipe close, overwriting the old
       value.

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

       TTeecchhnniiccaall NNoottee oonn tthhee SSyynnttaaxx ooff VVaarriiaabbllee NNaammeess

       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
       256 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 the perlmod manpage.

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

       All Perl variables 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.  A few other names are also exempt:

               ENV             STDIN
               INC             STDOUT
               ARGV            STDERR
               ARGVOUT
               SIG

27/Mar/1999            perl 5.005, patch 03                     1