Thanks everybody!
@Util - perfect! Exactly what I was looking for!
I really liked the way you created the clusters. Then it took me some time to understand the map-sort-map (unitl I found it in the cookbook) and the un-clustering (well, I didn't really understand that one, but can take it as given).

Not only did you solve my problem, but you also greatly enhanced my understanding of Perl and added to my toolbox of solutions to common problems!

One small note though:
Mapping like this: map  { [ $_->[0]->as_HTML, $_ ] } leads to problems when you have more tags in the dt element (some are links as well), thus it's better to map  { [ $_->[0]->as_text, $_ ] } or even to apply some more calculations on the text like lc and (at least in Germany) Umlaut considerations.

More than happy,
svenXY