Thanks Jan, I noticed that your module uses Hook::LexWrap too. So I was moving in the right direction after all. Today I found this other module Debug::Trace. I have a feeling TraceSubs is probably better with it's indented output representing call depth. Debug::Trace on the other hand doesn't require any changes in the source code. Are there any other differences?
I've never used Debug::Trace and it's ages since I used Debug::TraceSubs for the last time so I can't comment.
To minimize the changes to the code you can put all the Devel::TraceSubs related code into a separate file and add just require 'that_file.pl'; into the source, under the use statements. It's not perfect, but the $devel_tracesubs->trace() needs to run after all the subroutines you want to track were loaded and defined.
Jenda Enoch was right!
Enjoy the last years of Rome.