When running under a virtual host, Apache::Registry and other registry family handlers will compile each script into a separate package. The package name includes the name of the virtual host if the variable $Apache::Registry::NameWithVirtualHost is set to 1. This is the default behavior.
Under this setting, two virtual hosts can have two different scripts accessed via the same URI (e.g., /perl/guestbook.pl) without colliding with each other. Each virtual host will run its own version of the script.
However, if you run a big service and provide a set of identical scripts to many virtual hosts, you will want to have only one copy of each script compiled in memory. By default, each virtual host will create its own copy, so if you have 100 virtual hosts, you may end up with 100 copies of the same script compiled in memory, which is very wasteful. If this is the case, you can override the default behavior by setting the following directive in a startup file or in a <Perl>section:
$Apache::Registry::NameWithVirtualHost = 0;
But be careful: this makes sense only if you are sure that there are no other scripts with identical URIs but different content on different virtual hosts.
Users of mod_perl v1.15 are encouraged to upgrade to the latest stable version if this problem is encountered—it was solved starting with mod_perl v1.16.