Actually, testing those bits on the CPAN is more useful. There's a lot of stuff in the core, but it gets a lot of attention from a lot of smart people on a lot of platforms. It's not 100% tested yet, but it's getting there.

Some CPAN modules are used a lot, and they're tested... but there's a lot of stuff that's genuinely useful. A little feedback now and then is really helpful for authors. (It's especially nice when someone says, "This didn't work on my platform. Here's a patch!")