Having the luxury of time to consider it ;-) , here was my approach using index.

my @pos;

my $start = index($str, 'M');

while ($start != -1) {
    my $pos;
    my $i = 0;
    
    1 while ($pos = index($str, 'M', $start + $i)) == $start + $i++;
    
    push @pos, [$start, $start + $i-2];
    
    $start = $pos;
}
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Update - fix three lines

my $i = 1;
    
$i++ while ($pos = index($str, 'M', $start + $i)) == $start + $i;
    
push @pos, [$start, $start + $i-1];
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