Be sure to have the latest version of GD::SecurityImage (v1.66) and have a healthy libgd (not GD.pm, the C library itself). Also, if you have Image::Magick, you can use it to see if the problem is related to GD. However, you need to pass a valid ttf font parameter for Image::Magick. it'll be something like:

use GD::SecurityImage use_magick => 1;

__PACKAGE__->config->{captcha} = {
session_name => 'captcha_string',
new => {
width => 120,
height => 50,
lines => 1,
font     => "/absolute/path/to/your.ttf",
},
create => qw/ttf rect/,
particle => 100,
out => {force => 'jpeg'}
};
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