Provided the rendering engine can render into a pixmap instead of a
window at all, and that it can be used from Perl.
This limits you to the size of the browser window. Which is probably
what you want for a thumbnail, but I was thinking in a more general
way (sorry for not mentioning the shift in focus).
There is a plugin for Firefox called Screengrab which I use. It is able
to take a snapshot of the entire contents of the browser window (not the
window decorations, though). Its source code might provide some clues.
Alternatively, on Win32, I would try to use Win32::GuiTest to scroll the
window as necessary (it is not obvious to me how I would figure that
out), take repeated snapshots using IrfanView and stitch them
Sinan