O'Reilly has put up a sample chapter for the book 'Writting Apache Modules with Perl and C', which covers Apache::Registry.

Link to chapter
http://www.oreilly.com/catalog/wrapmod/chapter/ch04.html
Link to Apache::Registry section
http://www.oreilly.com/catalog/wrapmod/chapter/ch04.html#3278

It's really a good place to start learning about mod_perl, since it was written by the author of mod_perl and the author of CGI.pm.