Your tool has already been created on *nix (or cygwin). It's 'diff', 'diff' is an awsome little tool and fast. Just save the previous copy of what you want to compare to and 'diff prev_file current_file' It prints the differences between the 2.

If you want to persue the perl solution for academic reasons. Copy a previous file. Open the previous file and current file read them into arrays and compare the 2 arrays.

UNTESTED

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

open (FH,"<prev_file") or die;
my @prev = <FH>;
close FH;

open (FH,"<current") or die;
my @current = <FH>;
close FH;

foreach (0 .. $#prev) {
    print $prev[$_] if ($prev[$_] ne $current[$_]);
}
[download]

Don't recreate a good wheel - apropos is your friend

grep

grep> cd pub grep> more beer [download]