In the previous section I changed &Carp::format_arg to do something different. The general idea is very useful for debugging since I’m not only going to find bugs in the code that I write, but most often in the modules I use or in code that someone else wrote.

When I need to debug these things in other files, I want to add some debugging statements or change the code somehow to see what happens. However, I don’t want to change the original source files; whenever I do that I tend to make things worse no matter how careful I am to restore them to their original state. Whatever I do, I want to erase any damage I do and I don’t want it to affect anyone else.

I do something simple: copy the questionable module file to a new location. I set up a special directory for the debugging section just to ensure that my mangled versions of the modules won’t infect anything else. Once I do that, I set the PERL5LIB environment variable so Perl finds my mangled version first. When I’m done debugging, I can clear PERL5LIB to use the original versions again.

For instance, I recently needed to check the inner workings of Net::SMTP because I didn’t think it was handling the socket code correctly. I choose a directory to hold my copies, in this case ~/my_debug_lib, and set PERL5LIB to that path. I then create the directories I need to store the modified versions, then copy the module into it:

  $ export PERL5LIB=~/my_debug_lib
  $ mkdir -p ~/my_debug_lib/Net/
  $ cp `perldoc -l Net::SMTP` ~/my_debug_lib/Net/.

Now, I can edit ~/my_debug_lib/Net/SMTP.pm, run my code to see what happens, and work toward a solution. None of this has affected anyone else. I can do all the things I’ve already showed in this chapter, including inserting confess statements at the right places to get a quick dump of the call stack. Every time I wanted to investigate a new module, I copied it into my temporary debugging library directory.

Please check back next week for the conclusion to this article.