As for HTML and Perl source code: Once you start using UTF-8 here, you must not re-save the same files from text editors which do not yet support UTF-8, otherwise the extended characters in the source text get messed up.

There are unfortunately still quite a lot of programs which only support Latin-1 (iso-8859-1) encoding. In HTML, you could get around the problem with the classic solution of the nineties: writing HTML entities, like é for é etc.

For the same backward compatibility reason, I usually avoid any non-ASCII character (i.e. ord($char)>127) in filenames.