#! /usr/local/bin/perl

$GOOD="5000000000000009";

    # $CC="$GOOD"; # FOR TESTING

$CC="5123345612319876";

    # pre-screen dates and total number of digits based
    # on CC type first.
    # then...
    # if returned value does not equal 0 the number
    # failed checksum test

if (&CheckDigit($CC)) {
  print "Invalid Credit Card Number\n" 
}else{
  print "Credit Card Number is Good\n" 
} 
exit;

###

sub CheckDigit {
  my ($card_number) = @_;
  my ($sum, $double, $pos, $digit);
  $sum = 0;
  $double = 0;
  foreach $pos (1 .. length($card_number)) {
    $digit = substr($card_number,-$pos,1);
    if ($double) {
      $double = 0;
      $digit += $digit;
      if ($digit > 9) {
        $digit -= 9;
      }
    } else {
      $double = 1;
    }
    $sum += $digit;
  }

  #
  # If test passes - return=0 (a "false" flag for if stmt)
  return($sum % 10);
}