I'm no expert, but based on the docs, it looks like you want something more like this:

   $mech->submit_form(
       form_number => 1,

       fields => { old  => 'Evaluation-Folder/Evaluation-Sub-Folder-1'
+, },
       button => 'Delete'
   );
[download]

It appears to be telling you there's no select called old to be found...

EDIT: I tried WWW::Mechanize on one of my own sites that I'm developing and all I can say is: thanks dude. This module rocks and I had never heard of it... Just wow. (Also, my code above appears to work as written.)