++ for a thoughtful idea, but I agree with some others that this probably wouldn't be very effective in avoiding trolls. I am somewhat vulnerable to trolls - I tend to respond to them, argue with them, and ultimately waste my time with them. I almost never downvote them - I prefer to upvote worthwhile posts, rather than to downvote useless ones. I seldom "waste" a vote. What I do waste is time - so unless you could magically make troll posts vanish, chances are I'm going to get sucked into them and argue - even if the post has a flashing icon saying, "TROLL".

Another objection I have is simply that posts often get downvoted because they make an unpopular - but not unreasonable - claim. What the votes measure is not the worth of a post, but simply its popularity. Some well-reasoned, respectfully worded posts have been downvoted simply because they discussed something that was unpopular. Chances are the OP's post itself is among those. I haven't yet voted for it, but I wouldn't be shocked to see a significant number of downvotes.

A troll alert is likely to incorrectly identify at least some of these reasonable, but unpopular, posts. I think this would hurt Perl Monks in the long run.

Finally, it seems that Anonymous Monk's posts don't really get all that much downvoting. Limiting the maximum negative rep to -9 doesn't seem like it would accomplish much. Most of the really bad nodes are by known Monks.