There's the additional issue that modules have shorter development cycles than the core language. So e.g. the File::Basename module bundled with the distribution might be updated before the current perl interpreter is.
That means I find myself going to CPAN for most modules anyway, for installation and to find regular updates. So why not take the final step and instead of bundling a lot of modules with perl, change the install process so it initializes the CPAN or CPANPLUS shell and installs bundles directly from CPAN?
Having good quality and topical bundles is a prerequisite in that case, of course. That may be something we need to invest in.