I've written large web apps that scaled across clusters of machines using mod_perl handlers, CGI::Application, and HTML::Mason. I've written many unit tests for the web UI with Mechanize. My co-workers have written some with Selenium to test JS code. I've given some talks on these subjects at OSCON over the years. What are you trying to figure out?

One thing I can tell you is that in terms of scaling, the web framework you choose is probably your least important decision. What actually matters is I/O, so it's all about databases and caching tools.