So after not having used the perl module Petal for a few years, I find I'm a bit rusty. Rusty in the software industry means using google a lot to find answers. And when I searched to find how to iterate over an array, and use the array as hash keys or sub keys, I was interested to find a question that I had posted to the Petal mailing list 2 years ago.
Unfortunately, at that time no-one helped me, but I figured it out. Fortunately I posted the answer myself.
Just goes to show how important it is to fill out an answer on a forum/list. How many times can you find people with the same problems as you, but no answers....?