I do not know if you can do this, with Perl or otherwise, but it looks to me that it goes fully against the grain of what HTML ought to be.

I always thought that the HTML has nothing to do with the actual rendering of the page and that on widely different types of output devices, the page renders indeed widely different. Perhaps someone is looking at it with a text-only browser and someone else has a high definition graphical workstation with super-large screen. If you look through a windowed application, you can resize your window and the rendering-engine should recalculate how it shows the page in your window.

Try it with this page: resize the window and see at which column the nodelets start. I can resize my window over a rather large range before I get horizontal scroll-bars.

What the effects can be of the application of individual CSS-files or font-sizes or ..., makes your task more of a guessing game.

So the best you can hope for is to know what the X/Y coordinates are on a particular system with a particular window size.