Once a module goes into the core of Perl 5, it stays there forever, for good or evil. Having written tests for a chunk of core modules, some of that code isn't very good. (In 2004, we know a lot more about good Perl 5 programming than anyone did in 1995, so please don't read into that sentence more than I intend.) Some of the interfaces are terrible.
Will those modules ever go away? Unlikely. Backwards compatibility is important. Will those modules ever have interface-compatible replacements? Maybe someday, but how will you convince people to use them?
I wonder, idly, if every core module became dual-lived, distributed in the core and on the CPAN, if someone could write Bundle::CoreModules, keep it up to date, and the core could grow smaller.
Yes, there's still a problem with those people who compile Perl themselves apart from vendor packages, but I'd be surprised if those people don't have the technical saavy to download and install such a bundle somehow.
That leaves shops who refuse to install anything not distributed with the core. It's too bad that business stupidity isn't more lethal (to businesses).
There may be drawbacks I don't see right now, but the idle thought has a lot of appeal at the moment.