Sure, mod_perl is quite good at this. I've worked on or with a number of of mod_perl-based site: Yahoo! Pipes, their internal configuration management system, and at my job at WhiteHat Security. In addition, Shopzilla and Ticketmaster are both Perl-based (I *think* they're mod_perl, but I don't know the codebases).
In short, Perl can be a good alternative for a high-performance website. You will need to add caching and multiple servers to handle a really big load. It's better to architect for this at the start rather than later, so database replication and somebody like nginx or Squid to distribute requests are already part of the first implementation; that way you won't have to shoehorn them in later,which costs time when you don't have it (production is falling over under load, and you realize that the problem is that your architecture doesn't allow you to expand).If they're already in place, it should be a relatively simple exercise to just add more capacity and nodes to keep up with demand.
