This to my mind is one of the big reasons Perl is losing out to PHP, because PHP is very quick and easy to get results with, and Perl takes effort.

In my mind, that's more about deployment than it is about learning syntax. People don't learn syntax. They copy and paste and tweak example code until it appears to behave appropriately.

Plack helps with Perl's deployment. Plack helps a lot.

I suggested that when understood the system is easy to extend, modify and maintain, and saves programmer time.

That'd be easier to judge if you were to provide more examples.

I also suggested that given the state of current hardware the extra processing overhead is largely irrelevant...

The hardware specs you quoted are pretty heavyweight though, especially for a site measuring traffic in the hundreds of thousands of hits per month. When a persistent pure-Perl Plack-based server like Twiggy or Starman can serve thousands of hits per second, the bottleneck gets back to inefficiencies in processing, IO latency, and database traffic.

Even so, moving to a persistent process model is probably the best thing you can do. (I prefer Plack to mod_perl myself these days for many reasons, and not only performance.)