Hi, I' m the author of GD::SecurityImage. Image::Magick was an early addition and it was a request from someone else. It does not use the full power of Image::Magick (which is far more powerful than GD, but slower) and is merely a compatibility layer. You can use several different fonts and randomly changing styles/particles/scramble to make it hard for OCRs. But it may be possible to crack.. I don't know (I got zero feedback on this subject).
I didn't like Authen::Captcha' s approach with "graphic letters" and it's lack of plug-ability. It has it's own flat file database, while I prefer DB & sessions. I also like Tim Toady ;)
I've added a sample part to show the generated images: GD::SecurityImage