Odd. I found the perl debugger especially valuable for deciphering other people's modules, especially for bugs in CPAN modules.

For example, about four or five years ago, there was a bug in one of the PDF modules we were using; the author had clearly forgotten the /g modify on one of his regexps, so that only the first occurance of a given class of special characters was being properly escaped.

I didn't have to grasp the entire PDF specification, nor the author's intent in implementing that specification, in order to detect and correct the bug, because I had a nice debugger to guide me, and show me what was going on inside all the tricky objects and methods and twisty maze of function calls.

Especially with OO, where a function can return objects from different classes, each of which has their own special version of a method call, it's very nice to know exactly what function is being called, and which data is being passed, when all the code says is "call method X with object Y".