Perl: What is the meaning of ;\$ inside the arg list of a sub declaration? -

here part of program generated s2p, program builds perl script performs particular sed command.

# getsargv: read input line argument (default: $_). #           move on next input file, , reset eof flag $iseof. sub getsargv(;\$){     $argref = @_ ? shift() : \$_;     while( $iseof || ! defined( $$argref = <arg> ) ){         close( arg );         return 0 unless @argv;         $file = shift( @argv );         open( arg, "<$file" )         || die( "$0: can't open $file reading ($!)\n" );         $iseof = 0;     }     1; } 

i couldn't find explains sub signature on perlsub.

the code work quite well, too.

it's prototype. particular prototype forces parser accept usages

getsargv() 

and

getsargv($scalar) 

and causes them compiled as

&getsargv() 

and

&getsargv(\$scalar)