To further the above response, you could use Perl to parse the proxy logs, generate a report from those logs, web-enable queries against the logs, or a myriad of other tasks that Perl may be useful for. As the proxy itself, Perl is probably not the first choice I would look at. There are other tools out there that are much better - squid comes to mind as one of them.

If you also want to ensure that your clients do not use anything except the proxy, ensure that your firewall only allows the proxy through. You will also need to ensure that other ports cannot be used to bypass the proxy through the firewall.