...but there is no easy way for Perl to find out what encoding a filename uses.

Not sure why Perl would have to find out the encoding of filenames. Last time I needed such a thing, I would have been perfectly happy to have a pragma - something like use filenames "UTF-8", or so - in order to manually specify the encoding being used with filenames on the system in question.

I mean, no need to auto-determine anything here. Perl doesn't try to determine the encoding of file's contents either, or the encoding of the script source... For this, we have IO layers and use utf8; (for the source) or use encoding "...";. In other words, here it's also in the responsibility of the programmer to specifiy all encodings being used.  Why should filenames be handled differently?