Although GTK+ is portable to a lot of platforms, and Perl is portable to a lot of platforms, the Perl-GTK+ interface doesn't work on Win32 (at least, that was the situation when I tried it ~6 months ago).

Tk or Wx are probably the best bets for portable GUIs under Perl.

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I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated