Slightly off topic, but I believe amazon.com does use Perl. I know imdb.com and slashdot.org, two extremely busy sites, do. It just goes to show that your setup is more important than the technology you use.

Back to the question: Since you already know Perl, you'll probably be able to dive into PHP with no problems. However, if you're familiar with Perl there are a few things in PHP that might trip you up, like their "Perl compatible" regular expressions, which take a bit of getting used to.