But in perl there are Threads, ithreads, threads, 5005threads

There are only ithreads!

1. "Threads" and "5005threads" were the same, and are long since gone.

   Ignore these and the module that implemented them: Threads.

2. "threads" and "ithreads" are the same thing.
   • "ithreads" means interpreter threads.

     That is, each system thread runs an independant copy of the interpreter, with all non-explicitly shared variables created by that thread, being confined to that interpreter.

     Ie. If you don't explicitly share it, each thread runs (almost) as a separate process would.

   • "threads" is the name of the module threads that implements ithreads.

     To share data between threads, you must use threads::shared also.

     The are some other ways also, but enough for now.