Not reserved. They're used for pragmata, but not reserved.

perlmodlib.pod and perlstyle.pod:

"Perl informally reserves lowercase module names for 'pragma' modules like integer and strict."

perltoot.pod:

"You can look at other object-based, struct-like overrides of core functions in the 5.004 release of Perl in File::stat, Net::hostent, Net::netent, Net::protoent, Net::servent, Time::gmtime, Time::localtime, User::grent, and User::pwent. These modules have a final component that's all lowercase, by convention reserved for compiler pragmas, because they affect the compilation and change a builtin function."

"Reserved by convention" is arguably a contradiction, but it looks like lower case names are OK as long as you use a namespace that starts with upper case - say for example DBIx:: :-)