It's an interesting idea, but I think you're right in the drawbacks you point out yourself. In addition, the points others have made about using "code" tags for general formatting purposes make it unworkable as well.

Perhaps strongly nudging posters in to the

use strict;
use diagnostics

way of thinking before posting code could eliminate some of the problems you run in to, let the OP catch some of their own errors, and perhaps help the OP's in the long run as well in understanding the errors.

There is no emoticon for what I'm feeling now.