Neat problem. Here is my initial crack at it:

use strict;
use Data::Dumper;

my $string ='    aaaa    bbbbccccccccbbbb   aaaabbbbbcdddddddddddddddd
+dddd';
my @bah;

my $old_pos = 0;
while ($string =~ /((.)\2*)/g) {
   push @bah, [$2,$old_pos,length($1)];
   $old_pos = pos($string);
}

print Dumper(\@bah);
[download]

I am really interested as to how the benchmarking turns out. Let us know.

Update: It is late, but I just realized i would probably benchmark all the code that turns up on this post anyhow. Once again interesting problem.

Updated Again: Changed the \2+ to \2* or it would have failed when there was a series one character long.

-enlil