Joost's given you a solution using regexps. Here's one using split:

my $str = '-----MMMMM------IIIII----MMM---OOOO---I---MMMM-----';

my ( $start, $end );
for ( split /(-+)/, $str ) {
    my $char = substr $_, 0, 1;
    next unless length; # WTF does split return an empty string at the
+ start
    if ( $char ne '-' ) {
        print $start+1,
          length > 1
            ? ( "-", $start + length )
            : "",
          ": $char\n"
    }
    $start += length;
}
[download]

Output:

6-10: M
17-21: I
26-28: M
32-35: O
39: I
43-46: M
[download]