Not much to add with etcshadow and revdiablo hitting all the points, but...

In answer to why you need PHP/mod_perl/Mason... your need may be driven by your developers. If you're the developer, you can do as little or as much as you'd like with any/all of them. But, if you're also creating a system on which others will develop, you may want to provide some options.

For example, PHP is fairly useful for a lot of canned, sysadmin-useful applications. And, the non-Perl, non-Unix admins with whom I work seem to be more comfortable with PHP, even compared to CGI. Also, IMHO, dyed-in-the-wool ASP devleopers tend to take to PHP pretty quickly, so you might be able to leverage existing ASP programming resources with PHP (or Apache::ASP... I've never used it, so I can't comment).

So, I suppose the main point is to let your development needs be your guide. If you have none beyond your own edification, try them all!