use strict;
use warnings;

my ($s1, $s2, $l, $i, $c1, $c2, @m);

$s1 = 'ATACCGGC'; $s1 .= 'ATTTT'x1000000;
$s2 = 'ATTCCGGG'; $s2 .= 'ATTTT'x1000000;

for $i (0..(length($s1)-1)) {
    $c1 = substr($s1, $i, 1);
    $c2 = substr($s2, $i, 1);

    push @m, [$i, $c1, $c2]
        if $c1 ne $c2;
}

print (($#m+1) . ' mismatches with target at position(s) ' . join(', ', map { $_->[0] } @m) . ' (' . join(', ', map { $_->[2].'->'.$_->[1] } @m) . ')');