#!/usr/bin/perl

# http://perlmonks.org/?node_id=1144211

use strict;
use warnings;

@ARGV or @ARGV = qw( ATGGAT ACGGAG );
@ARGV == 2 or die "usage: $0 seq seq\n";

my ($left, $right) = @ARGV;

$_ = $left ^ $right;

print "Pos ", $+[0], ": ", substr($left, $-[0], 1),
  ' => ', substr($right, $-[0], 1), "\n" while /[^\0]/g;