Full documentation of predefined variables: perlvar
$_ - The default input and pattern-searching space
$a, $b - Special package variables when using sort(), see "sort" in perlfunc
$<digits> ($1, $2, ...) - Contains the subpattern from the corresponding set of capturing parentheses from the last successful pattern match, not counting patterns matched in nested blocks that have been exited already
$& - The string matched by the last successful pattern match (not counting any matches hidden within a BLOCK or eval() enclosed by the current BLOCK)
${^MATCH} - This is similar to $& ($MATCH) except that it does not incur the performance penalty associated with that variable, and is only guaranteed to return a defined value when the pattern was compiled or executed with the /p modifier
$` - 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)
${^PREMATCH} - This is similar to $` ($PREMATCH) except that it does not incur the performance penalty associated with that variable, and is only guaranteed to return a defined value when the pattern was compiled or executed with the /p modifier
$' - 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)
${^POSTMATCH} - This is similar to $' ($POSTMATCH) except that it does not incur the performance penalty associated with that variable, and is only guaranteed to return a defined value when the pattern was compiled or executed with the /p modifier
$+ - The text matched by the last bracket of the last successful search pattern
$^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
@+ - This array holds the offsets of the ends of the last successful submatches in the currently active dynamic scope
%+ - Similar to @+, the %+ hash allows access to the named capture buffers, should they exist, in the last successful match in the currently active dynamic scope
$. - Current line number for the last filehandle accessed
$/ - The input record separator, newline by default
$| - If set to nonzero, forces a flush right away and after every write or print on the currently selected output channel
$, - The output field separator for the print operator
$\ - The output record separator for the print operator
$" - This is like $, except that it applies to array and slice values interpolated into a double-quoted string (or similar interpreted string)
$; - The subscript separator for multidimensional array emulation
$% - The current page number of the currently selected output channel
$= - The current page length (printable lines) of the currently selected output channel
$- - The number of lines left on the page of the currently selected output channel
@- - $-[0] is the offset of the start of the last successful match
%- - Similar to %+, this variable allows access to the named capture buffers in the last successful match in the currently active dynamic scope
$~ - The name of the current report format for the currently selected output channel
$^ - The name of the current top-of-page format for the currently selected output channel
$: - The current set of characters after which a string may be broken to fill continuation fields (starting with ^) in a format
$^L - What formats output as a form feed
$^A - The current value of the write() accumulator for format() lines
$? - The status returned by the last pipe close, backtick (``) command, successful call to wait() or waitpid(), or from the system() operator
${^CHILD_ERROR_NATIVE} - The native status returned by the last pipe close, backtick (``) command, successful call to wait() or waitpid(), or from the system() operator
${^ENCODING} - The object reference to the Encode object that is used to convert the source code to Unicode
$! - 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
%! - Each element of %! has a true value only if $! is set to that value
$^E - Error information specific to the current operating system
$@ - The Perl syntax error message from the last eval() operator
$$ - The process number of the Perl running this script
$< - The real uid of this process
$> - The effective uid of this process
$( - The real gid of this process
$) - The effective gid of this process
$0 - Contains the name of the program being executed
$[ - The index of the first element in an array, and of the first character in a substring
$] - The version + patchlevel / 1000 of the Perl interpreter
$^C - The current value of the flag associated with the -c switch
$^D - The current value of the debugging flags
${^RE_DEBUG_FLAGS} - The current value of the regex debugging flags
${^RE_TRIE_MAXBUF} - Controls how certain regex optimisations are applied and how much memory they utilize
$^F - The maximum system file descriptor, ordinarily 2. System file descriptors are passed to exec()ed processes, while higher file descriptors are not
$^H - This variable contains compile-time hints for the Perl interpreter
%^H - 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
$^M - Perl can use the contents of $^M as an emergency memory pool after die()ing
$^O - The name of the operating system under which this copy of Perl was built, as determined during the configuration process
${^OPEN} - An internal variable used by PerlIO
$^P - The internal variable for debugging support
$^R - The result of evaluation of the last successful (?{ code }) regular expression assertion (see perlre)
$^S - Current state of the interpreter
$^T - The time at which the program began running, in seconds since the epoch (beginning of 1970)
${^TAINT} - Reflects if taint mode is on or off
${^UNICODE} - Reflects certain Unicode settings of Perl
${^UTF8CACHE} - This variable controls the state of the internal UTF-8 offset caching code
${^UTF8LOCALE} - This variable indicates whether a UTF-8 locale was detected by perl at startup
$^V - The revision, version, and subversion of the Perl interpreter, represented as a version object
$^W - The current value of the warning switch, initially true if -w was used, false otherwise, but directly modifiable
${^WARNING_BITS} - The current set of warning checks enabled by the use warnings pragma
${^WIN32_SLOPPY_STAT} - If this variable is set to a true value, then stat() on Windows will not try to open the file
$^X - The name used to execute the current copy of Perl, from C's argv[0] or (where supported) /proc/self/exe
ARGV - The special filehandle that iterates over command-line filenames 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
ARGVOUT - The special filehandle that points to the currently open output file when doing edit-in-place processing with -i
@F - The array @F contains the fields of each line read in when autosplit mode is turned on
@INC - The array @INC contains the list of places that the do EXPR, require, or use constructs look for their library files
@_ - Within a subroutine the array @_ contains the parameters passed to that subroutine
%INC - The hash %INC contains entries for each filename included via the do, require, or use operators
$ENV{expr} - The hash %ENV contains your current environment
$SIG{expr} - The hash %SIG contains signal handlers for signals