"When Perl is run as a webserver module, one instance of the interpreter is started with the webserver and multiple threads of execution are used"

Correct me if I'm wrong but with mod_perl (Apache), we have one instance of the Perl interpreter for each thread/process of the WebServer, and not the apposite.

The key word is "started". While each mod_perl process has their own copy of the perl interpreter you only suffer the time expense of starting up perl and loading modules the one time. After that it's just cheap forks.