Thanks Corion

I post the whole working code again for those who would try this module.

use Mojolicious::Lite;
plugin 'TagHelpers';
plugin 'captcha' => {
    session_name => 'session_string',
    out => { force => 'jpeg' },
    particle => [ 0, 0 ],
    create   => [qw/normal rect/],
    new      => {
        rnd_data => [ '0' .. '9', 'A', 'B', 'C' ],
        width    => 300,
        height   => 100,
        lines    => 1,
        angle    => 90,
        gd_font  => 'Giant',

        thickness => 2,

    }
};

get '/surv';

get '/captcha.jpg' => sub {
    my $c = shift;
    $c->render( $c->stash( data => $c->create_captcha, format => 'jpeg
+' ) );
};

post '/surv' => sub {
    my $c   = shift;
    my $app = $c->app;
    $app->log->debug( "tried: " . $c->req->param('captcha_value') );
    $app->log->debug( "Value: " . $c->session->{'session_string'} );
    if ( $c->validate_captcha( $c->req->param('captcha_value') ) ) {
        $app->log->debug( "Hi there " . $c->param('first_name') );
    }
    else {
        $app->log->debug("could be a bot here");
    }

};

app->start;
__DATA__

@@ surv.html.ep
%= form_for surv => (method => 'POST') => begin
<br>
%= text_field 'first_name'
<br>
%= image 'captcha.jpg', alt => 'A captcha'
<br>
%= text_field 'captcha_value' 
<br>
[download]

The characters are somewhat too small in the image, I should explore the use of TrueType font.

Here is a an example with ttf. The ttf file is in the same folder then the script.

plugin 'captcha' => {
    session_name => 'session_string',

    out => { force => 'jpeg' },
    particle => [ 0, 0 ],
  create   => [qw/ttf box #80C0F0 #0F644B/],  
    new      => {
        rnd_data => [ '0' .. '9', 'A' .. 'H', 'a'.. 'h' ],
        width    => 120,
        height   => 60,
        lines    => 1,
        angle    => 0,
        font => 'arial.ttf',
       ptsize => 24,

    }
};
[download]