Writing Apache Modules with Perl and C, by
Lincoln Stein and Doug MacEachern (O'Reilly);
selected chapters available online at http://www.modperl.com/
mod_perl Developer's Cookbook, by
Geoffrey Young, Paul Lindner, and Randy Kobes (Sams Publishing);
selected chapters available online at http://www.modperlcookbook.org/.
CGI Programming with Perl, by Scott Guelich,
Shishir Gundavaram, Gunther Birznieks (O'Reilly)