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my %before = (U => 'I', D => 'O');
my %after; @after{ keys %before } = reverse values %before;
...
    tr/DU/MM/ or tr/s/I/;
    return $_
}

use Test::More;

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