Actually, (and oddly), GD::SecurityImage also has support for an Image::Magick backend as well. You're probably better off using the GD backend. The amount of support for fiddling with the image'd text is pretty low though, and probably fairly easy to crack. The included font with the package is pretty well suited for this application, though.

I personally have used this module for CAPTCHA work on my web app and also have had requests for an audio-version of the CAPTCHA image. I've been looking at the Authen::PluggableCaptcha module, but there's no audio support for it, yet. Sigh.