Writing a web application is usually a repetitive process. For every page, you create a new script. This script performs essentially the same tasks as the other scripts in the application such as connecting to a database, getting some input from the user, and producing a web page as the result. This style of application design results in a structure, as shown in the following screenshot:

It may be useful to note that this concerns traditional non-framework designs, as there are more web frameworks for Perl.

Building an application with this sort of design is certainly not a difficult process, but the constant repetition can be prone to error. If you decide to change your database, you will have to update the database connection string in each file. If you happen to forget one obscure file, you might not be aware until your users start sending you angry e-mails.

Of course, this problem is not limited to the database connection strings. As the next diagram shows, repeated elements are there everywhere. As each page generates its own HTML, a request from your web designer to update the layout of a page will involve having to dig through your Perl source code (potentially breaking database queries or other logic), just to change some HTML. In addition, whenever you make a change that affects your entire site or application, you'll have to make the change a number of times—once for each page. Similarly, fixing one bug will entail copying the fix to every other place. This is inconvenient, a waste of time, and just plain boring.

Catalyst is an open source Perl-based Model-View-Controller (MVC) framework that aims to solve this problem by reorganizing your web application.