Another tool to debug your Plack application is InteractiveDebugger

You can dive into your code, explore your stack and see all variables in each frame.
And execute arbitrary code at some level!

Just try it:
plackup idebug-demo.psgi
firefox http://127.0.0.1:5000/

I'm was playing with PSGI spec and Plack last few days and found Debug middleware just amazing:

You should just try it:

plackup debug-demo.psgi
firefox http://127.0.0.1:5000