Perl pragma for deprecating the inclusion of a module in core

"deprecate" pragma simplifies the maintenance of dual-life modules that will no longer be included in the Perl core in a future Perl release, but are still included currently. The purpose of the pragma is to alert users to the status of such a module by issuing a warning that encourages them to install the module from CPAN, so that a future upgrade to a perl which omits the module will not break their code.