BOL

There are some variables which have a predefined and special meaning in Perl. They are the variables that use punctuation characters after the usual variable indicator ($, @, or %), such as $_ ( explained below ).

Special Symbols – File handlers

$@ Perl error string

$! Error number from C, ‘errno’

$^E Extended OS error info, such as ‘CDROM tray not closed’

$? Exit status from last process

$AGRV – name of current file

@ARGV – command line arguments

$ARGV – special file handle for command line filenames

$. – current line number

$/ - input line delimiter

$\ - output line delimiter

$% - current page number

$&/${^MATCH} – last successful matching string

$`/${^PREMATCH} – the string preceding the last matching string

$’/${^POSTMATCH} – the string following the last matching string

$1, $2, … - matching groups in the parentheses in pattern

More at https://www.tutorialspoint.com/perl/perl_special_variables.htm