You say "Thank you for agreeing that Perl will need many programmers and maintainers still for many years to come" ?

Seriously though, it depends on what you want to achieve. It doesn't sound like he wants to be persuaded, so either take the job (and bill for learning/converting things to PHP), or don't. The customer is always right, even when they're wrong. After all, if you're just doing a contract job, they have to maintain it, or find people to, so it's their problem.

C.