in reply to Libraries for web based UI (complex one)
Unless you're talking Tk or some other window manager interface, you can't do UI like that with Perl...? I'm sure you know this so maybe I don't understand the question.
If you're looking for the most Desktop-like JS lib for web stuff, it's ExtJS. It was a fork from YUI originally, IIRC, and it's feature rich if a little on the large side. There are two examples of using it with Cat here: Getting started with ExtJS screen library and Catalyst with Ext+Ajax: Editable Data Grids.
FWIW, there are a couple of really hot jQuery (my pet lib) plugins that do some of the same things (like live data tables and dojo-style comboboxes). Though jQuery plays nicely with ExtJS and other libs so you can mix and match.
(update: downcased k in Tk; linked jQuery.)