Though I've never used UPLOAD_HOOK myself, I think what you are describing is more or less the easiest way of creating a real progress indicator.

One thing that may simplify the system a bit is using a session system to store the upload_id, so you don't have to know it before opening the popup. Something like <form ... onclick="window.open('popup.pl','progress');"> will do then.

You can also fake the whole thing with an animated gif in a popup-window (that's what the Exchange webmail interface does).

-- 
Joost       downtime n. The period during which a system
            is error-free and immune from user input.
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