BDD says that testing (disregarding semantic affections) should also be key to design.

So does TDD. It's a design activity. That's why the final step (before "repeat") in TDD is "refactor".

What I seek are perl idioms for from-scratch application design, in perl, that implement acceptance test cases.

I'm afraid I don't appreciate the difference between using existing Perl automated testing modules and techniques and idioms to write acceptance tests versus unit tests. The scope of the test and what it tests varies, but the automation and libraries and harnesses and runners and emitters don't.

The real question is "How can I express a necessary condition of the software in the terms most naturally applicable to the problem domain, not coupled to any particular low-level design of the implementation?" I believe that FIT is one approach worth your time. It's not perfect, but it's a very good way to start to consider the problem. You can, of course, do the same thing with Test::More and its ilk (though if you find yourself needing to create many mock objects to make it work, I may start to suspect you've gone into the weeds).