The problem is that Win32 seems to be the only platform that offers an Unicode-aware file system API.
And as far as I know, all perls compiled for Windows still use the "ANSI" API to access the file system (i.e. CreateFileA instead of CreateFileW). Completely switching Perl to use the "wide" API may have "interesting" side effects. A very obvious one would be readdir returning Unicode strings instead of byte strings. Will existing code be able to handle that?
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)