I think Perl philosphy has two main problems with this idea:
TIMTOWTDI (though you leave options open, the basic philosophy still has problems. And a standard API would have problems with plug-in GUIS, because the way X-forms looks at windows (for example), and the way GTK does, are quite different)
Cross platform? Java's not half as cross platform as the hype, and Perl has wider boundries yet. Tk and GTK are both striving for cross platform, and I think they both beat out Java, but they can't come close to everywhere Perl runs.