It depends on the OS, and how your user environment is set up, and which perl version you're using -- e.g. on Redhat 9, the default user environment includes "utf8" as part of the locale setting, and perl 5.8.0 in that setup would treat the locale setting as the default encoding for all i/o. A lot of people got tripped up by that, so as of 5.8.1, perl doesn't behave that way anymore.

You just have to study the relevant details for your particular setup. The perlunicode and perllocale man pages you have there should help to figure out what's going on.