They have names like 'charnames', 'open', and 'vmsish' and seem to be core Perl modules

They are Core pragmas.

Can someone explain to me what the all lowercase CPAN module name designates?

From perlmod...

Module names are also capitalized unless they're functioning as pragmas; pragmas are in effect compiler directives, and are sometimes called "pragmatic modules" (or even "pragmata" if you're a classicist).