I'm guessing that you are talking about a website.

In theory, you could track unique visits on a per login per computer basis using an ActiveX control. This would have practically nothing to do with Perl though. Said control would have to pass data to the server, and you could use Perl on the server side, but that's about as far as it goes.

It isn't a very good solution though. ActiveX only works in Internet Explorer (and related browsers) on Windows (so say goodbye to anyone using a Mac, Linux, Firefox, a mobile phone, etc, etc). Also, allowing the install of an ActiveX control allows a website to do practically anything to a system - asking for that level of trust just to track unique visitors isn't sensible.

Saner approaches would be based on cookies (with allowances being made for users who don't accept the cookie) or logins (which can tie a session to an email account, or some other piece of evidence of uniqueness (not that an email account is an infallible example of such).