Might as well ask how many licks to the center of a tootsie pop 😉
The world shall never know.
Personally, I believe PHP and perl are two different animals completely.
PHP has blown away perl for HTTP scripting as far as ease of use and raw speed.
Perl is IMHO, the tool of choice for shell scripting and process functions that run independent of the user front end, but not something I would tie a front end to on the web since it operates much slower than PHP over an HTTP request.
And you may disagree with this take, but considering just how popular the combination of Linux - Apache - MySQL - PHP has become, you might want to consider as to why.
