I have this conversation a lot.
I always pose the client another question.
"Are you willing to pay every other client i have had a fee for the discoveries i have made while working with perl?"
Perl is a hammer. If someone builds a house with a hammer, the hammer is not the house. Nails are functions of perl. The nails i sink into your project belong to you, but the swing of the hammer will always be mine. And the nails belong to everybody, thanks to Larry.
It's like Newton claiming ownership for gravity because he wrote it down first.

just my 2 cents,