While the style guidelines would be the same as any non-Tk application, the design will be different.

While design patterns are not applicable for all applications, for GUI applications, adopting a good design pattern (I recommend the Model-View-Controller (MVC) pattern) can improve your general sanity considerably.

I would also strongly using OO Perl if you don't already. Trying to write a GUI app without proper encapsulation results in monolithic testaments to spaghetti and long sleepless nights in front of a monitor.

With both these things in mind, I would recommend creating an object diagram and a sample class diagram, presenting them to the development team and incorporating them into your set of artifacts, and performing design reviews periodically to make sure folks conform to the standard model you choose.

If you don't use UML right now, allow me to offer a plug: I find Dia a simple but stable and highly useful UML diagramming tool if you need to create some diagrams but don't need the full feature set of something like Rational Rose or Object Domain (which don't forward or reverse engineer Perl anyway). I think it comes default with most Red Hat installs these days ....

Hope this helps :-)