The prefork pragma is intended to allow module writers to optimise
module loading for both scenarios with as little additional code as
possible.

The prefork.pm is intended to serve as a central and optional
marshallingM point for state detection (are we running in procedural or
pre-forkingM mode) and to act as a relatively light-weight module
loader.

WWW: http://search.cpan.org/dist/prefork/