I wondered about the same thing a while ago, and was given the following "magic incantation" for defaulting to UTF-8 for both opening files and the STD* filehandles:

use open IO => ':utf8', ':std';
[download]

That said, there are reasons why Unicode isn't the default; see e.g. why no default unicode?. tchrist's insights on Unicode (already mentioned above) are also worth reading.