You can put a meta-refresh tag in the page, which'll cause it to re-request every so often. Also, you can make what's called a non-parsed-header cgi page... that is, the web server continues to stream something to the client continuously (as opposed to a fixed content-length).

Of course, if you were referring to how you would do this on the server-side, you can look into something that implements a "tail -f" type of behavior... either by, say, opening a pipe from "tail -f $file |", and doing non-blocking IO on it (see IO::Select), or good chance there's already a module that does something like this. (Maybe called Tail::Follow? I don't know. Check CPAN.)