sub luhn {
    $r=$t=0;
    split//,pop;
    $r.=$_[$_]*(1+($_+$#_)%2)for+0..@_;
    $r=~s/(.)/$t+=$1/eg;
    !chop$t
}



##</code><code>##

sub luhn2 {
    $_=$i=0;
    split//,pop;
    $_.=(1+$i++%2)*pop while@_;
    s/(.)/+$1/g;
    $_=eval;
    !chop
}