I've used PHP for most of my web database work, but my new task set has me using Perl a lot more again, so I've got some experience to comment with.

I can't address Java, because I've never used it, although one of my programmers swears by it. Ditto Python.

Perl (IMHO) is syntactically harder than PHP. Expect to have a learning curve here, and be prepared to buy a copy of every o'Reilly book released on Perl. That's the only way I survive Perl, and, even at that, I need Perl Monks troops to come to my rescue occasionally. Perl is (IMHO) more powerful than PHP, especially when you compare the module archives available for each.

I would agree wth the poster who argued for writing your abstraction layers in Perl as opposed to C. I used to wing it on projects with different languages for different parts of the job, but, over the long term, maintenance will bite you. This is going to be a big, long-term project, and I'd evaluate it carefully in that context.