want to write function that will return largest pandigital prime number based on passed parameter

That's a tall order. My first instinct would be to browse on the internet, and see whether someone already has a list. I don't find such a list on Sloane. It does have an entry for Pandigital primes, with some Mathematica code to generate them. But you'll have to generate millions of them to even get to the smallest number findLargest can return.

If I were to calculate them in Perl, I'd use PDL.