$ perl othercode.pl | perl thiscode.pl

##</code><code>##

$ perl thiscode.pl SOMEFILE.txt

##</code><code>##

#!/usr/bin/perl -w
use strict;

# thiscode.pl

my $file = $ARGV[0];
my @lines;

if($file)
 {@lines = `cat $file`;}
 else
 {@lines = <STDIN>;}  # Update: Also, can I do better than 'slurping' here?

foreach (@lines) {
  #Do my stuff here
}