I use a simple rule of thumb: if I can find it under lib, it's a core module, but if under site/lib, where all additional modules get installed by default, it's not...

But the latter doesn't mean it didn't come with perl — and this is actually your main concern. ActivePerl, likely the distibution for which installing extra modules is seen as problematic (though not actually for technical reasons... ;-)), comes with quite a few extra installed modules, such as XML::Parser, HTML::Parser, and LWP. You surely wouldn't want to miss out on those for no good reason.

So, the safest is still to check if a standard install comes with them.

Even so, if a module is worth it, I'd still use it even if it didn't come with perl.

If anything, it's possible to bundle the additional modules, or the whole install, as a PAR archive.