I'm not sure what you mean by "code verification", but I've been very successful with both CAPTCHA and blacklisting. I blacklist both by IP address and user agent, since some user agents are clearly not human users or responsible spiders.

Blocking by IP is pretty easy stuff. Grab $ENV{REMOTE_ADDR} and compare that value to your list of blacklisted hosts. You could get really crafty and use, say, Net::IP::Match or Net::IP::Match::XS to get out of writing a lot of code. With one of those two modules, you could simply do:

  if(match_ip($ENV{'REMOTE_ADDR'}, @blacklist)) {
    # tell user to take a hike
  }
[download]

You'd have to roll your own to do a match on user agent strings, but that's dirt simple stuff.

CAPTCHA need not use images. If you have a long list of questions that can easily be answered by a cabbage, you can select from that list at random and ask the user to type his response.

Have fun with it!