As said here, there are at least 3 multiplatform GUIs: Perl/Tk, wxPerl, Prima.

wxPerl looks like the windows manager of the O.S., the others have their own look & feel.
Perl/Tk is a mature product, well documented (it has a book and a newsgroup), wxPerl is young but it's growing up (it is strongly OO), and Prima is focused on 2D image processing