Small steps. Just using some simple functional constructs like `map` and `filter` will do wonders for your code. You don't have to start writing everything in a purely functional manner to benefit from it.
I'm asking more about things like tail-call recursion/trampolining, function decoration -- making the "architecture" of your program more functional, rather than just a few lines here or there.
Functional code isn't just good for concurrency, it also helps when trying to reason about lists and associative containers at a higher level.
It starts by teaching a reader moderately familiar with Perl functional concepts, and then it deep dives into these concepts and iteratively applies them to example programs to make them better.