Do you know of any GUI apps for Win32 or OSX implemented entirely in Perl?

For Win32, there's Win32::GUI. You might also want to take a look at Tk, which works fine on Win32 and, I presume, on OSX, too. I would think that those options are much simpler than embedding ... assuming they are adequate.

But I get the feeling you're seeking a GUI app that performs some specific function(s) ... in which case my answer is "no" ... largely because I'm unclear as to what the specific function(s) is/are :-)

Cheers,
Rob