Man pages matching name posix::atfork in section 3pm:

POSIX::AtFork.3pm.en	debian 10 / libposix-atfork-perl
POSIX::AtFork.3pm.en	debian 11 / libposix-atfork-perl
POSIX::AtFork.3pm.en	debian 12 / libposix-atfork-perl