( bigwig | 2011. 03. 05., szo – 15:44 )

Talán:

my ( $c1, $c2 );
while (<STDIN>) {
    next if $line > $.;
    $c2 = substr $_, $offset, 1;
    if ( $line == $. ) {
        $c1 = $c2;
        next;
    }
    if ( $c1 ne $c2 ) {
        print;
        last;
    }
}