Benchmark: timing 10 iterations of Algorithm::LUHN, Tweaked Algorithm::LUHN, bdlightner...
Algorithm::LUHN:  6 wallclock secs ( 6.63 usr +  0.00 sys =  6.63 CPU) @  1.51/s (n=10)
Tweaked Algorithm::LUHN:  5 wallclock secs ( 5.25 usr +  0.00 sys =  5.25 CPU) @  1.90/s (n=10)
bdlightner:  8 wallclock secs ( 7.81 usr +  0.00 sys =  7.81 CPU) @  1.28/s (n=10)

# tweaked Algorithm::LUHN with "use integer" and some simplification
sub cd3 {
  use integer;
  my $total = 0;
  my $flip = 1;
  foreach my $c (reverse split //, shift) {
    $c *= 2 unless $flip = !$flip;
    while ($c) {
      $total += $c % 10;
      $c = $c / 10;
    }
  }
  return (10 - $total % 10) % 10;
}
[download]