The module reuses the same CGI::Application object instance for several requests, which seems like a good idea at first, but as it turns out (cmp. firstname.lastname@example.org... ff.) CGI::Application and especially some plugins are not designed to be used in this manner. You might even run into severe security issues when using CGI::Application::Plugin::Authentication, like sending an authentication cookie which was intended for person A, who has just logged in, to person B, among similar phenomena.
So my recommendation would be: Do not use CGI::Application::FastCGI, but just build your instance script on CGI::Fast, as described here: www.cgi-app.org/index.cgi?FastCGI