Well two things:
In IIS, you need to require user authentication, otherwise, the machines will not send the authentication information, and then, if you have the Perl IIS mappings you can use the variable $Request->ServerVariables("REMOTE_USER")->Item which returns the authenticated user. That works in the actually ASP pages only.
Running as a script, you may want to look into Win32::Eventlog and then send all page access to the EventLog running on your server.