perl -le'$m=1;$s+=$_*($m^=2)for(split(//,$ARGV[0]));
print(($s%10)?"BAD":"OK");'

##</code><code>##

sub check_upc
{
  my($upc)=@_;
  my $m = 1;
  for(split(//,$upc)) {
    $s += $_ * ($m^=2);
  }
  return (($s%10)?"Invalid checkdigit; should be @{[10-($s-chop($upc))%10]}"
                 :undef);
}