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]

# WTF does split return an empty string at the start

Because you can interpret "-" as being two empty strings separated by "-". IMHO the real WTF is why does it remove the empty trailing string.
"What should it profit a man, if he should win a flame war, yet lose his cool?"

use strict;
use warnings;

my $str="-----MMMMM------IIIII----MMM---OOOO---I---MMMM-----";
my @my_splitted_str=split(//,$str);

# add a sentinel value after the other characters, so
# the end of the last range is properly detected
push @my_splitted_str, "-";

my $range = ""; # we are not initially in a range
my $start_of_range;

for my $index ( 0 .. $#my_splitted_str ) {
    # if we are in a range, see if it ends now
    if ( $range && $range ne $my_splitted_str[$index] ) {
        # the previous character was the last of this range
        my $end_of_range = $index - 1;
        if ($end_of_range == $start_of_range) {
            print $start_of_range + 1, ":", $range, "\n";
        }
        else {
            print $start_of_range + 1, "-", $end_of_range + 1, ":", $r
+ange, "\n";
        }
        $range = "";
    }

    # see if this is the start of a range
    if (! $range && $my_splitted_str[$index] ne "-") {
        $start_of_range = $index;
        $range = $my_splitted_str[$index];
    } 
}
[download]