Is utf8::downgrade always guaranteed to produce a string with the required encoding for any environment though?

No, it just convert UTF-8 back to Latin1 (or EBCDIC in case you're using an A/400 or S/390). It simply looks like your file names are encoded in Latin1 on your drives. IIRC windows 2000 used Latin1, while Windows XP uses utf8. I may be wrong though because I'm a strict Un*x/Linux guy :)