#!perl

use strict;
use warnings;

undef $/;

for my $file (@ARGV) {
     open F, $file or die $!;
     $_ = <F>;
     close F;

     $changed = 0;

     # put your code here, mark $changed if you made changes
     # For example:
     #  $changed = s/this/that/g;

     if ($changed) {
          open F, ">$file" or die $!;
          print F;
          close F;
     }
}