Yes, you can do it in one line (instead of three). The following counts all non-dash characters, without modifying your original string : 
use strict;
use warnings;

my $str='ABC---DR----EEEEEGGGG-GRE-RED----KKKK---';
my $len_seq = () = $str =~ /[^-]/g;

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This is a FAQ: 

perldoc -q count

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See also:

In reply to Re: Write code using less lines by toolic
in thread Write code using less lines by Anonymous Monk

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