Obviously the next question you'll ask is: "Is this all I need to know about mod_perl?".
The answer is: "Yes and No".
The Yes part:
Just like with Perl, you have to know very little about mod_perl to do really cool stuff. The presented setup allows you to run your visitor counters and guest book much faster and amaze your friends, usually without changing a single line of code.
The No part:
A 50 times improvement in guest book response times is great, but when you deploy a very heavy service with thousands of concurrent users, taking into account a high level competition between similar web services, a delay of a few milliseconds might cost you a customer and probably many of them.
Of course when you test a single script and you are the only user, you don't really care about squeezing yet another millisecond from response time, but it becomes a real issue when these milliseconds add up at the production site, with hundreds of users concurrently generating requests to various scripts on your site. Users aren't merciful nowadays--if there is another even less fancier site that provides the same service but a little bit faster, chances are that they will go over there.
Testing your scripts on an unloaded machine can be very misleading, Everything might seem so perfect. But when you move them into a production machine, things don't behave as well as they did on your development box. Many times you just run out of memory on very busy services. You need to learn how to optimize your code to use less memory and how to make the memory shared.
Debugging is something people prefer not to talk about, since the process can be very tedious at times. Learning how to make the debugging process simpler and efficient is a must if you consider yourself a web programmer. This task is especially not so straightforward when debugging CGI scripts, and even more complicated with mod_perl. Unless you know how, and then it suddenly becomes easy.
mod_perl has many features unavailable under mod_cgi when working with databases. Among others the most important are persistent connections.
You have to know how to keep your service running non-stop and be able to recover fast if there are any problems.
Finally, the most important thing is the Apache-Perl API, which allows you to do anything with a received request, even intervene in every stage of request processing. This gives you great flexibility and lets you create things you couldn't dream about with plain mod_cgi.
There are many more things to learn about mod_perl and web programming in general. In future articles I'll talk in details about all these issues.