The Perl Journal (http://www.tpj.com/) had an article on DBI in issue #5, and an article on mod_perl in issue #9. Both are worth reading if you're interested in these topics.
Anyone interested in using databases on the Web should read Philip Greenspun's Database-Backed Web Sites (published by ZD Press), which is an interesting and instructive description of how to create web sites which use databases. The book is currently out of print, but is available on the Web at http://www.photo.net/wtr/dead-trees/.
Source, binaries, and documentation for MySQL is available at http://www.tcx.se/. MySQL is available in RPM format for users running Red Hat Linux.
DBI is available from CPAN at http://www.perl.com/CPAN/modules/by-module/DBI/. Installation of the DBI code and documentation is described in the INSTALL document that comes with the package.
Also, DBDs for a number of databases are available on the Internet. The most up-to-date list of DBDs is at the DBI home page, http://www.hermetica.com/technologia/perl/DBI/.
Finally, if you are interested in learning more about databases, I have found some good books on the subject. For starters, try The Practical SQL Handbook by Judith S. Bowman, Sandra L. Emerson and Marcy Darnovsky, published by Addison-Wesley. For more advanced practical advice, try SQL for Smarties: Advanced SQL Programming by Joe Celko, published by Morgan Kaufmann. Finally, for a longer and more academic (but very readable) discussion of the topic, look into Database: Principles, Programming, Performance by Patrick O'Neil, published by Morgan Kaufmann.
All listings referred to in this article are available by anonymous download in the file ftp://ftp.ssc.com/pub/lj/listings/issue52/2991.tgz.