I believe the best practice is still doing it on case-by-case basis, at least when dealing with external data, since you never know what you'll get. Utf8 might be widespread but it's not like the other encodings are not used. As for automatic support, I guess this is as far as you can really go:

use utf8;
use open ':encoding(UTF-8)';
[download]

This will allow you to write unicode strings into the source and automatically handle encoding/decoding on read/write in new filehandles. Mind that if you read data from databases, external APIs or whatever other source that is not a filehandle, you will still need to set the specific options or decode it yourself, it will all the depend on the library used. Unfortunately, there is no one magic setting which will set everything to utf8.