You can save a little typing and also be a little more comprehensive with respect to "anything not a dash" by using the c flag (complement) of tr, see Quote and Quote like Operators.

knoppix@Microknoppix:~$ perl -E '
> $str = q{ABC---DR----EEEEEGGGG-GRE-RED----KKKK---};
> $len_seq = $str =~ tr{-}{}c;
> say $len_seq;'
24
knoppix@Microknoppix:~$
[download]

I hope this is of interest.