Big site with its own dedicated (virtual) server that you can configure and maintain yourself: Perl with mod_perl.

Site on shared bulk hosting where you have to choose between mod_php and mod_cgi: PHP if dynamic programming is necessary for every page, otherwise static pregenerated content with CGI scripts for the less often accessed dynamic pages.

Amazon.com, a huge high-traffic makes heavy use of Perl, but thousands of small web shops are built with PHP. Just the purpose of your site-to-be is no indication for the language to use. I think the most important thing is how skilled the programmers involved are or want to be. PHP is very limiting, and it'll forever keep its users at a beginner's level. Perl on the other hand has a steep learning curve, but allows one to reach a level at which you're no longer scripting or writing dynamic templates, but are actually programming a complex system much more easily.

If you start learning both PHP and Perl at the same time and spend equal time learning them, very quickly will you have become a PHP expert knowing all there is to know. At that point, your Perl skills will be considered merely intermediate. Once you reach Perl expert level, you start laughing at PHP :)