I haven't tried it myself yet (at least, not in Perl), but this sounds like a good case for using Mmap. The idea would be that you set yourself a comfortable buffer size (a few meg, or whatever makes sense for the GUI), and you map this amount of the big file into the scalar that gets put into the Text widget.

When the user scrolls the widget to a point outside the current mapping window, remap the scalar to the appropriate offset in the file. Assuming that it actually works, it ought to be pretty fast, and could probably be built into a callback function from the scroller widget.

(Unless of course you happen to be using a system that doesn't support mmap, in which case this is a useless idea.)