It all depends. What are you going to do? How much requests are you going to serve at peak time? How many servers? How much memory are you going to use? How heavy will your requests be? How fast are they (that is, can they quickly return an answer, or are you tying up resources cause you first have to fetch data from a database?)

There are many E-commerce sites that run well on modperl. There are many sites that don't, but could have. No doubt there are many E-commerce sites that do run on modperl, and perform poorly. And the most successful ones will hit limits 6 times a day, and 10 times on Sundays. One day it's the database, and when that is fixed, it'll be the network, and when that is fixed, it'll be the framework, and if that is fixed, we start all over again.