This is my first attempt at TDD in Perl. I'm using Test::Simple and nothing more complicated than ok. My approach to coding is to write the test, check that it fails and then write the code to make is pass. I'm trying to write the code from the bottom up, so that the first thing I did was to create the Excel instance. Then I accessed the code modules and made sure that I was getting the sheet names and line counts back. Then, getting sick of closing orphan instances manually, I wrote the code to close the created Excel instance. Then I wrote code to open the output files. And so on.
I'm not sure if this answers your question "how are you contemplating the process of “testing”", but what I want is a battery of tests that mean that, when I make a change, I can have some reassurance that I haven't made things worse.