When the data structures are a little complex (at least for the (not necessarily) intended audience ;--) I pepper the code with calls to Data::Dumper (actually with Data::Denter) that can be activated in debug mode, whether it is by setting a variable, an environment variable or just by uncommenting the print Dumper $HoHoA; statement. This way figuring out what's in the variable is not too hard for the maintainer. I don't usually use them myself actually, I prefer the wonderful x $HoH in the debugger, which works wonders, especially as you can then try print $HoH{foo}->{bar}; and see what comes out.

In any case not knowing how to deal with a HoH shows quite a limited grasp of the language (and of programing in general as a matter of fact). It might be worth mentioning to your co-worker that knowing how to deal with complex data structures is really required if he wants to be called a programer (you could maybe be a little more diplomatic than that though ;--).