In my mind, there are goot reasons for it. The scripts are provided by users of a webpage. While Perl is the swiss army knife of programming languages, in this case that versatility makes in unusuable - it's practically impossible to make it really secure while allowing it to interact with the webserver through a defined API.
While this is by no means a guarantee that it will be safe and secure, using a limited functionality sandbox makes it much easier for a small team (in my case: one man team) to keep an overview of what is and isn't possible for the user. I'm pretty sure this is a matter for discussion and possible flame wars - and i'm pretty sure that a larger team could come up with a better solution.
Please, don't understand this as anything else than stating my point of view and explaining the reasons for why i'm tending towards LUA - i'm not trying to start a flamewar*.
* Although we could if you like. I'm prefering IRC in that case, since i have an IRC bot for that. No, no, wait, it still uses Net::IRC, i should really rewrite that thing sometime soon... (bleep), before taking over the world i really need to hire some henchmen...
Don't use '#ff0000':
use Acme::AutoColor; my $redcolor = RED();
All colors subject to change without notice.