mod_perl Developer's Cookbook, by Geoffrey Young, Paul Lindner, and Randy Kobes (Sams Publishing). Selected chapters and code examples available online from http://www.modperlcookbook.org/.
For more information about signal handling, refer to the perlipc manpage
GET and POST request methods are explained in section 9 of RFC 2068, "Hypertext Transfer Protocol—HTTP/1.1"
RFC 2965 specifies the HTTP State Management Mechanism, which describes three new headers, Cookie, Cookie2, and Set-Cookie2, that carry state information between participating origin servers and user agents
The cookie specification can be viewed at http://home.netscape.com/newsref/std/cookie_spec.html
BCP 44, RFC 2964, "Use of HTTP State Management," is an important adjunct to the cookie specification itself
Cookie Central (http://www.cookiecentral.com/) is another good resource for information about cookies
"Design Patterns: Singletons," by Brian D. Foy (The Perl Review, Volume 0, Issue 1), available at http://www.theperlreview.com/.