Plack::App::CGIBin via Apache and mod_fastcgi - CGI script not found

Question

I'd guess you need to change

builder {
    mount "/plack" => $app;
};

to

builder {
    mount "/" => $app;
};

because your alias is removing /plack/

or just change

# URL to be handled by FastCGI
Alias /plack/ /tmp/placktest.fcgi/

to

# URL to be handled by FastCGI
Alias /plack/ /tmp/placktest.fcgi/plack/

After all it does not appear to be a Perl question, but a System Administrator one, about Apache configuration.