sub luhn {
    my $total = 0;
    for my $i ( 0 .. length $_[0] ) {
        my $d = substr $_[0], $i, 1 );
        if( $i & 1 ) {
            $d *= 2;
            $s -=9 if $d > 10;
        }
        $total += $d;    
    }
    $total *= 9;
    return substr $total, -1
}