Select Code to Download

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

sub check_upc
{
...
  return (($s%10)?"Invalid checkdigit; should be @{[10-($s-chop($upc))
+%10]}"
                 :undef);
}