sub luhn {
    use integer;
    my $s = $_[ 0 ];
    my $total = 0;
    for my $i ( 0 .. 14 ) {
        my $d = substr( $s, $i, 1 );
        unless( $i & 1 ) {
            $d *= 2;
            $d -= 9 if $d > 9;
        }
        $total += $d;
    }
    $total *= 9;
    return chop $total;
}