Found the improved version of Peter Eisch's plugin attached to http://www.nntp.perl.org/group/perl.qpsmtpd/2006/03/msg4710.html.  This includes caching of the system load and adds a cache_time config parameter.
Bumped the plugin version number for the variant with caching.  Fixed a typo in the perldoc.
Minor perldoc fix.
Added warning about expense of forking/exec to determine system load.  Added SEE ALSO links to the postings containing the plugin and an alternate.