To me, this sounds less like a question about technology and more a question about psychology. So, why does your boss prefer PHP?
It could be that it's a comfort zone thing .. the boss has looked at PHP examples and looked at Perl examples, and the PHP ones looked much clearer.
Either language will be easy to maintain -- you can use Perl with a page templating system (like Template::Toolkit, the one I use) and make the source code look very PHP-ish.
Frankly, it sounds like this is a simple enough project that you could do it in PHP no problems. If you've managed to learn Perl with any level of proficiency, I'm fairly sure PHP won't be much of a challenge.
And, of course, once finished you could try doing the same project in Perl and see how much better it is. ;)
Let us know how it all turns out.
Alex / talexb / Toronto
"Groklaw is the open-source mentality applied to legal research" ~ Linus Torvalds