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One of the tried and true ways of debugging is to sprinkle 'print "Here! $val\n"' around and see what happens or if the program even reaches that point. Simple and effective! Carp::REPL lets you take this a step further by starting an interactive Read-Eval-Print Loop (REPL) at an arbitrary point in your program.
I've taken this technique and mixed in a splash of Continuity to build an interactive web-based REPL and inspector for your application. The REPL lets you execute arbitrary code, the inspector gives you a GUI for traversing your callstack and manipulating your variables. It works on regular applications and CGI scripts too. You just add a single "inspect()" where you like and you'll be whackin' those bugs DOWN!
In this talk I'll show you how to use the tool and how it's built... and how you can add to it with plugins.