The very first thing to do in a case like this is to write a test plan, then write a whole bunch of tests. I leave all the rest of the tests to you.

Win8 Strawberry 5.8.9.5 (32)  Wed 09/07/2022 21:20:21
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

use Test::More;
use Test::NoWarnings;

# use Data::Dump qw(dd);  #  for debug

my @Tests = (

  'ALL these strings have replacements',
  [ 'UUUUUUUUIIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOO',
    'IIIIIIIIIIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOO',
    ],
  [ 'IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOOUUUUUUUU',
    'IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOOOOOOOOOO',
    ],
  [ 'IIIIIIIIIIIIIIIMMMMMMMMMMMUUUUUUUUMMMMMMMMMMMMMOOOO',
    'IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMOOOO',
    ],
  [ 'IIIIIIIUUUUUIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOUUUUUUUOO',
    'IIIIIIIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOOOOOOOO',
    ],
  [ 'IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOUUOO',
    'IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOOO',
    ],

  'NONE of these strings have replacements',
  [ ('IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMMMUUOO') x 2 ],

  );  # end @Tests

my @additional = qw(Test::NoWarnings);  # each of these adds 1 test
plan 'tests' => (scalar grep { ref eq 'ARRAY' } @Tests)
              + @additional;

VECTOR:
for my $ar_vector (@Tests) {

    if (not ref $ar_vector) {
        note $ar_vector;
        next VECTOR;
        }

    my ($input, $expected) = @$ar_vector;

    my $got = replace($input);

    is $got, $expected,
       "'$input' \n     '$got'"
       ;

    }  # end for VECTOR

sub replace {  # works

    my ($str,
        ) = @_;

    # only ONE or NONE of these replacements will be made.
    $str =~ s{ \A (U+) (?= ([IOPBM])) }      { $2 x length $1 }xmse  o
+r
    $str =~ s{ (?<= ([IO])) (U+) \z }        { $1 x length $2 }xmse  o
+r
    $str =~ s{ (?<= ([IOPBM])) (U+) (?= \1) }{ $1 x length $2 }xmseg ;

    return $str  # return string with possible replacements

    }  # end sub replace()

^Z
1..7
# ALL these strings have replacements
ok 1 - 'UUUUUUUUIIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOO'
#      'IIIIIIIIIIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOO'
ok 2 - 'IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOOUUUUUUUU'
#      'IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOOOOOOOOOO'
ok 3 - 'IIIIIIIIIIIIIIIMMMMMMMMMMMUUUUUUUUMMMMMMMMMMMMMOOOO'
#      'IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMOOOO'
ok 4 - 'IIIIIIIUUUUUIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOUUUUUUUOO'
#      'IIIIIIIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOOOOOOOO'
ok 5 - 'IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOUUOO'
#      'IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMOOOOOO'
# NONE of these strings have replacements
ok 6 - 'IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMMMUUOO'
#      'IIIIIIIIIIIIIIIMMMMMMMMMMMMMMMMMMMMMMMMMMUUOO'
ok 7 - no warnings
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See also How to ask better questions using Test::More and sample data.