You may be able to get rather better speed with some simplistic string manipulation. Consider:

  sub mul2 {
    my($str, $add) = @_;
    (my $carry = $str) =~ tr/0-9/0000011111/;
    (my $str2 = "0$str") =~ tr/0-9/0246802468/;
    my $result = $str2 | "$carry$add";
    $result =~ s/^0//;
    $result;
  }
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Hugo