The mod_limitipconn.c and Apache::LimitIPConn Apache modules: http://dominia.org/djao/limitipconn.html
These modules allow web server administrators to limit the number of simultaneous downloads permitted from a single IP address.
Chapter 9 ("Tuning Apache and mod_perl") in mod_perl Developer's Cookbook, by Geoffrey Young, Paul Lindner, and Randy Kobes (Sams Publishing)
Advanced Perl Programming, by Sriram Srinivasan (O'Reilly)