Special Variables

The $_ Special Variable

Perl's most popular variable is $_ . It is the default variable, which is always used when no explicit assignment is given (otherwise $_ has the value of the last assigned variable).

The following pairs are equivalent:

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

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

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

chop
chop($_)

It is useful to shorten loops, the following example prints the numbers from 1 to 10:

for(1..10) { print; }

List of Special Variables (for more detailed description see: I, II)


The following variables are global:
`$_`	The default input and pattern-searching space.
`$.`	The current input line number of the last filehandle that was read.
`$/`	The input record separator, newline by default. May be multicharacter.
`$,`	The output field separator for the print operator.
`$"`	The separator that joins elements of arrays interpolated in strings.
`$\`	The output record separator for the print operator.
`$?`	The status returned by the last `...` command, pipe close or system operator.
`$]`	The perl version number, e.g., `5.001`.
`$;`	The subscript separator for multidimensional array emulation. Default is "`\034`".
`$!`	If used in a numeric context, yields the current value of errno. If used in a string context, yields the corresponding error string.
`$@`	The Perl error message from the last eval or do EXPR command.
`$:`	The set of characters after which a string may be broken to fill continuation fields (starting with `^`) in a format.
`$0`	The name of the file containing the Perl script being executed. May be assigned to.
`$$`	The process ID of the currently executing Perl program. Altered (in the child process) by fork.
`$<`	The real user ID of this process.
`$>`	The effective user ID of this process.
`$(`	The real group ID of this process.
`$)`	The effective group ID of this process.
`$^A`	The accumulator for formline and write operations.
`$^D`	The debug flags as passed to perl using `-D`.
`$^F`	The highest system file descriptor, ordinarily 2.
`$^I`	In-place edit extension as passed to Perl using `-i`.
`$^L`	Formfeed character used in formats.
`$^P`	Internal debugging flag.
`$^T`	The time (as delivered by time) when the program started. This value is used by the file test operators `-M`, `-A` and `-C`.
`$^W`	The value of the `-w` option as passed to Perl.
`$^X`	The name by which the currently executing program was invoked.

The following variables are context dependent and need not be localized:
`$%`	The current page number of the currently selected output channel.
`$=`	The page length of the current output channel. Default is 60 lines.
`$-`	The number of lines remaining on the page.
`$~`	The name of the current report format.
`$^`	The name of the current top-of-page format.
`$\|`	If set to nonzero, forces a flush after every write or print on the currently selected output channel. Default is 0.
`$ARGV`	The name of the current file when reading from `<>`.

The following variables are always local to the current block:
`$&`	The string matched by the last successful pattern match.
$`	The string preceding what was matched by the last successful match.
`$'`	The string following what was matched by the last successful match.
`$+`	The last bracket matched by the last search pattern.
`$1`...`$9`...	Contain the subpatterns from the corresponding sets of parentheses in the last pattern successfully matched. `$10`... and up are only available if the match contained that many subpatterns.

Special Arrays

`@ARGV`	Contains the command-line arguments for the script (not including the command name).
`@EXPORT`	Names the methods a package exports by default.
`@EXPORT_OK`	Names the methods a package can export upon explicit request.
`@INC`	Contains the list of places to look for Perl scripts to be evaluated by the do FILENAME and require commands.
`@ISA`	List of base classes of a package.
`@_`	Parameter array for subroutines. Also used by split if not in array context.
`%ENV`	Contains the current environment.
`%INC`	List of files that have been included with require or do.
`%OVERLOAD`	Can be used to overload operators in a package.
`%SIG`	Used to set signal handlers for various signals.

Environment Variables

`HOME`	Used if chdir has no argument.
`LOGDIR`	Used if chdir has no argument and `HOME` is not set.
`PATH`	Used in executing subprocesses, and in finding the Perl script if `-S` is used.
`PERL5LIB`	A colon-separated list of directories to look in for Perl library files before looking in the standard library and the current directory.
`PERL5DB`	The command to get the debugger code. Defaults to `BEGIN { require 'perl5db.pl' }`.
`PERLLIB`	Used instead of `PERL5LIB` if the latter is not defined.

Environment variables are stored in the hash %ENV, they can be accessed as $ENV{'varname'}, example:

print "$ENV{'HOME'}\n";