I don't get why he would agree that Perl is powerful and efficient and still ask you to do the job in another language. And didn't you tell him that PHP was built by people looking at perl? PHP is barely even a language. Python, ruby, and other languages have neat stuff but Perl has the best balance of existing modules and being proven in the field while allowing you to do whatever you want in - as the customer says - in the most efficient way. In comparison with Perl or other real programming languages PHP is a toy, only fit for small jobs which is the opposite of what one normally thinks of as "having a future".

If it's a simple PHP job and you need the money fine but if it's a big project with little money I would say quit. I've done it and hated every step wishing I had perl. The customer specified PHP mainly because they wanted to be sure subpar engineers could edit the simple-looking PHP files. To be specific, the customer was a former coworker and project manager who cannot read perl and he wanted to be able to edit it. I am willing to bet that you are in the exact same situation. No sane client otherwise will say PHP is a language with a future.