my %map = map { $_ => $_ } 0..9;
sub check_digit {
  my @buf = reverse split //, shift;
  my $totalVal = 0;
  my $flip = 1;
  foreach my $c (@buf) {
    my $posVal = $map{$c};
    $posVal *= 2 unless $flip = !$flip;
    while ($posVal) {
      $totalVal += $posVal % 10;
      $posVal = int($posVal / 10);
    }
  }
  return (10 - $totalVal % 10) % 10;
}
# example use
for (401135000000000..401135000000099) {
   my $cd=check_digit($_);
   print "$_$cd\n";
}