The debug middleware offers a configurable set of panels that displays
information about the current request and response. The information is
generated only for responses with a status of 200 ('OK') and a
'Content-Type' that contains 'text/html' or 'application/xhtml+xml' and is
embedded in the HTML that is sent back to the browser. Also the code is
injected directly before the '</body>' tag so if there is no such tag, the
information will not be injected.

To enable the middleware, just use the Plack::Builder manpage as usual in
your '.psgi' file:

use Plack::Builder;