Here's another way to do it:

#!/usr/bin/env perl -l

use strict;
use warnings;

my $string = '---BB--BBB---BBB---B-B---';
print $string;

$string =~ y/B/M/;
print $string;

my @subs = qw{i o};
my $i = 1;
$string =~ s/(-+)/$subs[$i ^= 1] x length $1/eg;
print $string;
[download]

Output:

---BB--BBB---BBB---B-B---
---MM--MMM---MMM---M-M---
iiiMMooMMMiiiMMMoooMiMooo
[download]