With the aside that there is no such thing as "perfectly secure", you can get a long way with a CAPTCHA.

For those that may not be able to visit that site, a CAPTCHA is a program that can create and grade tests that are easy for a human to pass, but hard for a computer. Lots of big sites use them in the form of images that contain mangled strings -- enter the plain text that matches what's in the image, and you can reasonably assume the submitter is human.

Someone has even rolled a nice Perl implementation already.