Plack (and PSGI) is nice in the sense that you don't need to worry (that much) anymore about whether your script will be running under mod_perl or as CGI, or via HTTP::Server::Simple, and you still get to do fancy stuff like providing the page content through a callback. I found Plack nice when writing a webserver that did background page fetching through AnyEvent - I didn't have to look at how AnyEvent::HTTP does its thing but could simply use Dancer, which respects PSGI. In the end, I used Twiggy as the backend HTTP server.