Thank you.
I get the idea about "decoding" the byte string, so that Perl would know that this is to be treated as utf-8.
What I do still not quite get, is why the "-f" and the open() do fail when used on the original entry resulting from the readdir(). Anyone have an idea ?

(I'm also not quite sure if I'm pushing the right button to send this answer, but I guess I'll find out)