There are really two aspects to that question, which can and should be dealt with separately:
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There are many web-servers out there (including tiny Perl programs...), and many ways to deploy apps on those servers (FastCGI, mod_perl, etc.) It is highly desirable to be able to switch horses in mid-stream; hence the benefits of software-interface layers such as Plack. (“Design your app for Plack, and you can reconfigure Plack without having highly-pervasive changes in your application.”)
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Within the application itself, there is usually “a whole lot of treasure-chest and only a little treasure.” The chest is basically the same no matter what goes inside, so obtain a suitable chest “off the shelf.” Hence application frameworks, such as Catalyst (and dozens of others). Always try to avoid doing something that has already been done. You want to be able to confine your interests strictly to what is actually different about your application.
This approach has a subtle added benefit: it introduces a disciplined, and somewhat distant, perspective upon your own work.
