my %before = (U => 'I', D => 'O');
my %after; @after{ keys %before } = reverse values %before;
sub subst {
    local ($_) = @_;
    1 while s/(s+)(?=([DU]))/$before{$2} x length $1/e
          | s/(?<=([DU]))(s+)/$after{$1} x length $2/e;

    tr/DU/MM/ or tr/s/I/;
    return $_
}

##</code><code>##

use Test::More;

my %simple = (ss => 'II',
              ssUUss => 'IIMMOO',
              ssDDss => 'OOMMII',
              ssDDssUUss => 'OOMMIIMMOO',
              ssUUssDDss => 'IIMMOOMMII');

for my $input (sort keys %simple) {
    is subst($input), $simple{$input}, $input;
}