As with everyone else, I'm going to address the phase leading up to the question rather than the question. To take advantage of modern internet computing, you shouldn't be embedding your CSS in your HTML anyway. Ideally, it stays external which allows the client to download it once for your site, and not need to download it again. It can be better cached by proxies. It can be downloaded in parallel with the web page itself. Please. Don't.

That said, I've been guilty of embedding the CSS myself - but I'm feeling much better now.

Ok, I will answer your question now that I've gotten what I think is the important stuff out of the way. Yes, I'm sure that Perl and PHP can co-exist. Just like Perl and Java. Or shell and C. Or ... a whole host of other combinations. But you need to be careful that you don't end up needing to duplicate logic in each language. That's asking for problems - not only twice the likelihood of bugs, but that makes it infinitely more likely that you'll get the two out of sync when making changes/upgrades/bugfixes. This isn't a language thing - it's a cross-language thing. That said, just as it can be dangerous to mix, say, perl with a templating mini-language, as long as you're careful to keep each piece of logic in one and only one place, you'll probably be fine.