plXPCOM isn't a GUI kit - using Gecko as a GUI renderer is just one of its benefits. With plXPCOM it would be possible to create perl components that can be accessed and controlled via any Mozilla based browser or Gecko embedded application.
Inversely, it would be possible to access any XPCOM interface directly from Perl, in a similar manner as XS.
I dont know if you have worked with COM, but apart from the ActiveX/M$ part, it is a pretty awesome technology. XPCOM is simply a more secure cross-platform implementation.
I love being able to plug components developed by Apple, Adobe, Macromedia into my perl apps. Wx, Qt, Tk will never be this good. Mozilla already is.

Think about it, international companies develop for and with the Mozilla platform. Innovation follows it.

In regard to GUIs, its just more handwaving material;
XUL uses XML, CSS, DTDs - its contents can be skinned, internationaly localised, connected to an RDF datasource, and delivered across the network like a regular (or dynamic) webpage.
Anyone creating CGI applications or simply a designer who understands DHTML can immediately get to grips with the basics of GUI creation. GUIs are simpler to manage than their CGI/DHTML equivalent, and custom widgets are relatively easy to create.

Do you use Mozilla? Set your skin to "modern" and take a look at how a GUI should look. In comparison, Tk/Wx look decidedly amateurish.

Neato, a browser widget that can display colored text!

Perl has never made itself conducive to distributing applications, let alone GUI ones. It barely manages modest in-house projects. This is my no.1 gripe with my no.1 language.
XPCOM offers a framework that any language can plug into and represent itself from. This is great news for all scripting languages, but especially certain ones with a particular niche in string processing and data munging. Perl can do what its good at, and leave Gecko to make it look sexy.
A full-blown, slick, professional, cross-platform application written in Perl seems very realistic with XPCOM. Not with Tk.
When you said "mature", I think you meant "old". ;)

Perl not being bound to Mozilla, is like Perl not having Apache bindings. Its just plain wrong.


time was, I could move my arms like a bird and...

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