The goals of experimentation are different than the goals of testing. In test mode, you propose an output and verify that the actual output is "as expected". This, as you note, takes a lot of understanding of the module.

However, in experimental mode, you start with a goal and consider various possible inputs and incantations that might result in the desired output. It is exploratory, not predictive. For failures, Test::More::is prints out a lot more than just "not ok". In fact, it prints out the right result, so it can be a good way to "see what happens".

An alternative see-what-happens technique is the command line or a REPL (read-eval-print-loop). Both are very good tools for small clarifications. REPL's don't run into quoting problems like the command line does. However, both have several other limitations: repeated setup, lack of annotations, inability to repeat what you did last week, inability to repeat en-mass a batch of trials with different inputs, and so on.

$& - agreed. But that wasn't the point of the example and I apologize if it wasn't clear. The point was really to show

• how to test an incantation where the goal isn't simply the output of a subroutine (via eval)
• an experimental approach. The intent was to give a sampling of the kinds of mistakes one might make while feeling one's way to understanding how capturing and back-references work.
• being able to keep an annotated history of what does and doesn't work. A command line or REPL provides some short-term history, but no annotations you can go back to weeks later.
• the ability to repeat experiments en-masse for new input by encapsulating a set of tests in a subroutine.

Best, beth