ptkdb cpan module
is a well-known perl debugger using perl/Tk GUI.
Despite being single-file, it is is very powerful and has quite many features. I used it a lot earlier, and found it very useful.
Recently I've reworked it and
- made it use Tcl::Tk instead of perl/Tk, so it got modernized look-n-feel and faster
- removed a lot of really unused code and few unused features
- made lot of places faster
I haven't made online my changes, but am really planning to.
all in all, this module definitely deserves a good look into it - while perl documentation describes its debugging capabilities quite well, ptkdb is a good example of usage of these features.