What you need to know
You need to know Apache configuration, mod_perl basics and Perl

Introduction
mod_perl allows you to extend the Apache web server to process your specific requirements. This is done in Apache by having specific 'exit-points' in strategically placed stages of a request. By creating handlers in these specific points, you can change the way Apache handlers your transcations.

Handlers
Handlers can execute on server startup and at different stages in a transaction life cycle.
We will mostly be more interested in transaction handlers specifically:

• PerlPostReadRequestHandler

• PerlTransHandler

• PerlAccessHandler

• PerlAuthenHandler

• PerlResponseHandler

• PerlLogHandler

This handler executes when a request is received and all headers have been parsed. It executes only once in a transaction's life. You can change the headers that you received at this point, before the transcation is processed further.

PerlTransHandler
This executes when Apache translates the URI to a file on the server. This can be used to change the URI or the request method. If you made significant changes to your website file structure, you can use this to translate the URIs to your old website to a newer one.

PerlAccessHandler
If you want to implement a policy so that certain portions of your website is restricted to specific users, time or IP addresses, you can use this handler. It executes only once in the transaction. Your handler should return an Apache::FORBIDDEN or Apache::OK.

PerlAuthenHandler
You can use this to replace Apache authentication. If this returns an Apache::HTTP_UNAUTHORIZED return code, the browser normally pops up a login window.

PerlResponseHandler
This handler can generate specific responses based on the URI inputted. You can change the outgoing header and even the entire page.

PerlLogHandler
This handler allows you to improve logging. You can use this to write log information onto a database.

Filters
mod_perl 2.0 has filters. These filters allow you to have more control of the headers and pages in a transaction. You can use this to add or remove headers. You also use this to change contents in your response pages.

There are two filters:
PerlInputFilterHandler and PerlOutputFilterHandler.

The PerlInputFilterHandler is executed when the request is received.
The PerlOutputFilterHandler is executed before a page is sent back to the browser.

To know more about mod_perl Handlers, please read the mod_perl 2.0 or mod_perl 1.0 documentation.