Anonymous Monk,
You have not mentioned a few things I consider very important. Do you need to know to which file the extra line(s) came from? Can one file contain the same line more than once and is that relavent? Is order important?

I will give you both standard responses when this question is asked.

• Use a hash
• Use diff

#!/usr/bin/perl
use strict;
use warnings;

open (FILE1, '<', 'file1.txt') or die "Unable to open file1.txt for re
+ading : $!";
open (FILE2, '<', 'file2.txt') or die "Unable to open file2.txt for re
+ading : $!";

my %lines;
while ( <FILE1> ) {
    chomp;
    $lines{$_}++
}
while ( <FILE2> ) {
    chomp;
    $lines{$_}++
}
open (FILE3, '>', 'file3.txt') or die "Unable to open file3.txt for wr
+iting : $!";

for ( keys %lines ) {
    next if $lines{$_} > 1;
    print FILE3 "$_\n";
}
[download]

If you are not on a *nix system with diff or if you have not installed a *nix toolkit for Win32 you can find a pure perl implementation of diff here and sort here. You may also want to look into Perltidy. This question gets asked a lot so you may also want to look at our Q and A section in the future as well.

Cheers - L~R