# Pre-define a mapping for the alternate digits
# (rather than calculate sums in the loop every time):
my @LUHN10_map = ( 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 );

# Return 1 if input number passes LUHN-10
# checksum test, otherwise return 0.
sub LUHN10
{
   # NOTE: Assumes input consists only of digits.
   my $number = shift;
   my $sum = 0;
   my $length = length $number;

   # Sum the digits working from right to left.
   # Replace with mapped values for alternate
   # digits starting with second from right:
   while ( $length > 1 ) {
      $sum += chop $number;
      $sum += $LUHN10_map[ chop $number ];
      $length -= 2;
   }
   $sum += $number if $length;  # one digit left
                                # if length was odd

   # Result for valid number must end in 0:
   return ( ( $sum % 10 ) == 0 ) ? 1 : 0;
}