My first guess would be because PHP is being launched as a component of the webserver, while each time the perl script is called it is launched as an independent process. (I suspect PHP would not seem so much faster if it were being used as a CGI, rather than being loaded as a webserver module.) I would suggest if it is a significant concern perhaps a look at something such as mod_perl, where a perl interpreter would be loaded with the webserver and thus not have to be launched anew each time a call is made to that script.

My own experience with regards to mod_perl is limited, however, so I defer to those other monks herein who may have more experience with it.

Hope that helps...