Plack::App::Proxy is one of the example Plack application that has lot of contributions. It started as leedo's project but @hiratara and I have been contributing lots of patches and improvements, one of which is the big revamp to be able to take middleware to change the behavior of Proxy itself.
Now @frankcuny and @hiratara jointed their efforts to make the simple round-robin based LoadBalancer to pick which backend to connect to when App::Proxy is running as a reverse proxy. Pretty simple and nice. The commit is on github and will be available in the next CPAN release i believe.