Overall well done captcha generator, however it still could be improved.
First I missed some options, at least to define the letter spacing of the captcha. I tried different TTF fonts, all with a minimum size to be readable, but often the letters were placed outside of the image. It had to be enlarged to extreme sizes.
I think there is also need for some more distortion possibilities (e.g. to randomize particles or a setting to overlap letters, generally randomization is implemented very rarely).
Installation of another Perl module (GD::SecurityImage::AC) was required for me, because the Security-Image module comes without any captcha handler. Usually you are supposed to create that code yourself - which should be no problem if you are an experienced Perl programmer - but I am certainly not.
Besides those glitches there were no problems, installation was easy, there is a very good documentation available and I experienced fast and helpful support by the author. GD-SecurityImage worked perfectly together with the above mentioned AC module.