System Variables

$$     -   Yields the process id of the perl program
$@   -    Yields the error message STDERR
$|      -   Yields the corresponding System Error
@_   -    Stores all the arguments passed to a sub routine
$&    -    Part of  the string that matches the pattern
$`      -   String before the pattern match
$'       -   String after the pattern match
$^I    -     Backup file name's extension