You could write a plug-in to your favorite browser. Probably Perl is not going to apply here.

You could write a patch for an open source browser like Mozilla. Again, Perl is probably not going to be too much use here (I assume if you know enough C to understand the Mozilla source, you won't need to use Perl to implement the features you want).

You could write a browser in Perl-- or extend one that already exists. I've seen a few projects along these lines:
the sample browser included in the Tk::HTML module (which I've had mixed success with and which has pretty sparse documentation),
this one (I wrote this, it has almost no features, it gets ignored a lot and grows very slowly)
this Tk browser (I don't see any source for this one, but you might email the author)
this other Tk browser (the most advanced of the group)