I don't know if that rules out the possibility that the problem is where you said it is ...

Aaaah ... hang on ... in the Glib-1.301/t/9.t test script we find:

} elsif ($^O eq "MSWin32") {
    print "ok 12 # skip add_watch on win32\n";
    print "ok 13 # skip add_watch on win32\n";
    print "ok 14 # skip add_watch on win32\n";
}
[download]

This would suggest that add_watch() doesn't work on Windows, and you need to replace it with other code.
The Gtk2 developers probably have a good idea of what's needed, if no-one here comes forward with a solution.

Cheers,
Rob