IMO, that isn't good practice, just leave it on. There are two camps on this but when you look at the majority of modules on the CPAN, strict is on and IMO should be left on. If I see a module with it commented out I wonder why.

So the end user can do it however they'd like

What happens when a user sets these things in the module itself then you update the module and someone reinstalls it? Poof, their setting are gone. You should not expect people to set anything like that in the module itself. If your module comes with an external config file, fine, but don't expect people to do it within the module.

I know that it's port 25 because I was lazy in it. *shrug* I s'pose I flip it back to be a user-configurable var in a future release.

What? And wipe out my default when I upgrade? No thanks ;)

I don't particularly like using POSIX (or any other module, if possible) in a CPAN distribution

Some people like it, some people don't. But POSIX comes with Perl, so people have it already. May be overkill for this module, but I like POSIX for various reasons.

I didn't finish error ...

I gave some suggestions. If the method is a noop, take it out until you do something with it.

I'm not convinced the end user will fully understand what they're used for

So? Then they find out. Your docs should have an example of how to use it (like every other module that uses this technique).

My biggest nit is that you tell people to set various defaults in the module (and set a few yourself). This has problems, and can lead to later problems (like reinstalling newer versions, and having to edit an installed module to set/change these).

Cheers,
KM