Full documentation of predefined variables: perlvar
$_ - The default input and pattern-searching space
@_ - Within a subroutine the array @_ contains the parameters passed to that subroutine
$" - When an array or an array slice is interpolated into a double-quoted string or a similar context such as /.../, its elements are separated by this value
$$ - The process number of the Perl running this script
$( - The real gid of this process
$) - The effective gid of this process
$0 - Contains the name of the program being executed
$; - The subscript separator for multidimensional array emulation
$< - The real uid of this process
$> - The effective uid of this process
$a, $b - Special package variables when using sort(), see "sort" in perlfunc
$^C - The current value of the flag associated with the -c switch
$^D - The current value of the debugging flags
${^ENCODING} - The object reference to the Encode object that is used to convert the source code to Unicode
%ENV - The hash %ENV contains your current environment