CGI has only been removed from core, but now is (way) more actively maintained on CPAN. No loss there.
a2p and s2p may have been removed from CORE, but you can install App::a2p and App::s2p and now get maintained version. I agree that they give great tuturial value, but OTOH as such they are rather outdated.
Module::Build should never have been, just as cpanplus should never have been in CORE. Both served fine as a (bad) learning curve into the alternatives getting better, but neither ever deserved a CORE spot IMHO. Having both on CPAN is more than enough.
Enjoy, Have FUN! H.Merijn