reCAPTCHA version 1 is a hybrid mechanical turk and captcha that allows visitors
who complete the captcha to assist in the digitization of books.

From http://recaptcha.net/learnmore.html:

   reCAPTCHA improves the process of digitizing books by sending words that
   cannot be read by computers to the Web in the form of CAPTCHAs for
   humans to decipher. More specifically, each word that cannot be read
   correctly by OCR is placed on an image and used as a CAPTCHA. This is
   possible because most OCR programs alert you when a word cannot be read
   correctly.

version 1 of Perl implementation is modelled on the PHP interface that can be
found here:

http://recaptcha.net/plugins/php/

To use reCAPTCHA you need to register your site here:

https://www.google.com/recaptcha/admin/create


Version 2 is a new and eaasy to solve captcha that is
"easy for humans to solve, but hard for 'bots' and other malicious software"

• Igor Vlasenko

• perl-Captcha-reCaptcha