I highly recommend something like (but not necessarily) Config::IniFiles so that those users who care to muck about in the guts of the thing (the configuration thing, not the code thing) can, and any web interface you might create will be able to work with it and not screw things up. This keeps the users completely out of the code but gives them a very high degree of control.

Asking users to edit code is just begging for trouble. You cannot expect the end user to know that 'user@example.com' is okay, but "user@example.com" is not, or that $things and @things are two different things. It's much easier for you to use a non-eval'd configuration file, and much less scary for the end user, too.