Overall, nice discussion. To comment on one point you raised, though... you wrote:

    sub error {
        print $data; # oops, doesn't work!
    }
[download]

Well, it *does* work. If you try it, you'll see that it works just fine, because it's a lexical variable... lexical to the scope of the file. And the error sub is defined in the file, so $data is visible there.

That's not to say that it's recommended, of course, because it can definitely cause you problems. Particularly when using mod_perl. You'll get the dreaded "variable may not stay shared" message, because essentially you've defined a closure.

That said, though, good points.