Rather than focusing on how to do this in as few characters as possible, consider doing it in a manner that is clear

my $str = '---BBB--- ..... ';
my @bits = split /(B+)/, $str;
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produces @bits:

0: '---'
1: 'BBB'
2:'---'
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processing through @bits, you have to consider the possibility that the first character was a 'B', in which case $bits[0] will be B', in which case you convert to M's, otherwise the first part will convert to the odd pattern. Then a string of Bs, then the even pattern, and so on.