Running this under -w gives warnings on odd lengths for $number. You can shut them up with only a little slowdown with the following:

sub dkubb_LUHN10_2 {
  my $number = shift || return 0;
 
  my $sum;
  $sum += (chop($number) || 0)
          + $LUHN10_map[chop($number) || 0] 
    while "$number";
 
  ($sum % 10 == 0) || 0;
}
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