I seriously don't understand why DateTime has never made it into core; as far as I'm concerned Perl isn't installed until DateTime is installed.

But for a Big Distro of Useful Modules, I would argue for including anything that's ever been written up on the Advent calendar, any year, plus anything that's a dependency for CPAN modules by more than three other authors. (The latter criterion will catch quite a lot of the various Test:: and Net:: modules, plus ones that provide data structures, among other things. The former would catch stuff that's nifty and useful, such as WWW::Mechanize and Data::Dumper::Simple.) Obviously, you'd still exclude anything with outside non-Perl dependencies unless you happened to have those dependencies installed, and you'd exclude any platform-specific modules except on that platform.