I would go for PHP in your situation for a number of reasons. Firstly, you already have 10 years of perl and that knowledge isn't going to disappear just because you choose PHP this time round.

Secondly, precisely because you already know Perl so well, you should consider PHP so you've an additional skill to market.

Thirdly, it's always good to see things from the other side.

Finally, choosing Perl over PHP goes against your boss' wishes, which may give the impression you aren't adaptive.

Sidenote: I've coded a fairly extensive site in Perl. Recently, I did another site of a smaller scale in PHP. Because I was already familiar with some aspects of web logic, so it wasn't that difficult overall. I plan to try Python for a hobbyist site that I've been thinking of doing, just for the fun of it.