# returns error message on failure, undefined value on success
sub checkUPCrobot2
{
  # grab and immediately split upc into array, 1 char per element
  my @chars = split //, shift;

  # return error message if incorrect length
  return "should be 12 digits in length" if $#chars != 11;

  my ($even, $odd, $check);
  foreach (0..5) {
    $odd += shift @chars;
    $check = shift @chars;
    $even += $check;
  }
  my $mult = 3*$odd+$even-$check;
  my $chk2 = 10 - ($mult%10);
  return "invalid checkdigit...should be $chk2" if $check!=$chk2;
}