I don't think that that would be an ideal implementation to use all-wide internal strings, because on win32 conversion from utf8 into byte using system locale might differ from windows internal conversions. Because, well, windows locales can be strange. Sure, there are ANSI<->OEM<->WideChar conversion functions, but it would be a bad idea to require perl programmer to use these when the actual conversion is needed. It would be easier (and less problematic) to assume that system-dependent filename conversion rules will be not guaranteed to be identical to perl utf8<->byte conversion rules.

I'd say that a context-sensitive pp_readdir and friends would be just fine. In this case, when it expects utf8, on unix a simple SvUTF8_on() would be enough, and on win32 W-syscall instead of A-syscall would be issued. The question is how to determine the context.