I've been using Plack for a few days now, and I have to say, I really like it. I rewrote the base of my webshop platform to use it. The change wasn't really difficult and the code is a lot cleaner now.
Especially with the Middleware structure I was able to remove a lot of code from the main application. Middleware classes are classes that are called between the webserver and the main application. These classes have the opportunity to rewrite the request and the response classes for each request.
Because I use the same code for each webshop, but different databases for each customer, so I need a way to select the right database for a request. This is a great use for a Middleware class. The software selects and connects to the database based on the hostname in the request. The connection is then added to the environment hash. And the way back the database connection gets closed.
Furthermore there are already a few nice Middleware classes for logging and debugging. That is code you don't have to write. I really like it.