There seems to be a common misconception that all new technology must replace exisiting technology. In many applications twisted pair wires make more sense than fiber optics and the old technology lives next to the new technology. Radio didn't kill print. Television didn't kill radio.

We see this in economics. Many people are convinced that wealth creation means more poor people. But modern economies are not zero sum systems. Wealth creation creates a larger pie from which many enjoy larger portions than before. New software languages are often similar.

PHP didn't come in and replace applications. Like may other technologies had done before, it widened the use of programming as a solution to web site authors. Many people used PHP where they would have once stuck with HTML. The programming solutions pie got bigger. Everyone benefitted.

The customer is always right. If its PHP he wants and you are good at writing PHP programs, give him what he wants. Explain the limitations of PHP, but don't fall in love with Perl if it leads you to the poor house. It is okay to tell him he's wrong about Perl, just be careful not to maneuver him into a situation where you can say "I told you so." Perhaps you could introduce a PHP/Perl solution. Using each language where it is strongest.