I personally believe that while I would have agreed with you for years, as of late I've been seing web-based apps become more and more similar to "regular" GUIs, and more and more offered as controlling interfaces for other programs, that I'm not sure the distiction has the right to stay so neat any more...

One wild idea could be to use XUL as a framework, and indeed it may be not that wild after all, if you count the number of hits out of a simple cpan search: what's more, now that I notice, there's a module called XUL::Node which seems to be just about implementing this very idea!