sub checkUPCGrandFather2 {
    my $str = shift;
   
    return "should be 12 digits in length" if length ($str) != 12;
   
    my $t = 0;
    my $sum = 0;
   
    # loop through to sum even and adjusted odd chars
    for (0..10) {
        my $chr = substr $str, $_, 1;
        $sum += ($t ^= 1) ? $chr * 3 : $chr; 
    }
   
    # calculate correct check digit
    my $check = (10 - $sum % 10) % 10;
   
    # return error message if wrong check digit was initially given
    return ($check != substr $str, 11) ? "invalid checkdigit...should be $check" : 'Ok';
}