#!/usr/bin/perl

use strict;
use warnings;

while ( <> ) {
    ...
}

##</code><code>##

ls *.txt | my_script.pl

##</code><code>##

my_script.pl text.txt

##</code><code>##

#!/usr/bin/perl

use strict;
use warnings;

my @files;
if ( @ARGV ) {
    @files = map { -f $_ ? $_ : () } @ARGV;
}
else {
    while ( <> ) {
        chomp;
        push @files, $_;
    }
}