When in doubt add another table:

my @luhnmap_1 = (0,2,4,6,8,1,3,5,7,9);
my @luhnmap_2 = ();
for my $x (0..9) {
  for my $y (0..9) {
    @luhnmap_2[$y*10+$x]=$luhnmap_1[$y]+$x;
  }
}

sub LUHN10 {
  my $num = shift;
  my $sum = 0;
  while ($str = substr $num, -2, 2, "") {
    $sum+=$bigmap[$str];
  }
  return $sum % 10 ? 0 : 1;
}
[download]

As bonus, if you don't count the additional table code (aw, come on, please :) ), it's shorter! You can increase this speed some more if you want, but the only way to get a huge increase is to create a bigger table (instead of 10 or 100 elements, try 10,000!). Not sure if it's worth going the next step (the memory and upfront cost).

Enjoy,
Gryn