[<=]is a listop, so where I thought I was sending two arguments to
"array is sorted properly", actually I was sending just one, the equivalent of
[<=] @array, "array is sorted properly", and it is certainly not true that
8 <= "array is sorted properly". Problem solved!
Meanwhile, Hacker News pointed me to a very nice article by Peter Seibel on TDD, "Unit testing in Coders at Work". What's great is that it points out both cases where unit tests were absolutely crucial AND it points out great work done without them.
It also points out a hilarious example where Ron Jeffries goes completely off the rails trying to write a Suduko solver using TDD. Peter Seibel even coins a name for this "pattern": "Going in Circles Means You Don’t Know What You’re Doing".
Seriously, it's worth looking at Peter Norvig's solver to get an idea what a working Suduko solver looks like, and then check out those five posts of Jeffries'. I don't think I can say it better than Norvig did: "I think test-driven design is great. I do that a lot more than I used to do. But you can test all you want and if you don’t know how to approach the problem, you’re not going to get a solution."
BTW, Norvig's approach looks like it would be an absolute blast to code in Perl 6. If no one else has done it, I may take a stab once I've gotten a bit further on my current project...