The encryption is done through custom C executable and the execution of encrypted binary code can only be done through another C executable (dummy Perl interpreter) and it does in-memory execution like this.

perl_run(my_perl);
eval_pv(buffer, TRUE);
[download]

At the moment I don't have the time to Super Search, but I have a memory of a similar proposal made some time ago here. Given your quote below, you understand the caveats to this situation. Unless you post something that's repeatable, which would give people an opportunity to help find what you'd consider to be issues any "thoughts/suggestions/criticism" people can give may be of limited value to you.

"Ofcourse I am sure that it's almost impossible to completely secure the code from "too curious" eyes, but I tried to make things harder for them"

*sigh* read all the other threads and the threads they link, getting at the source code of anyting that calls perl_run and eval_pv is trivial