You're misunderstanding what Parallel::ForkManager does, I'm afraid. (As long as it actually does use fork, which seems likely from the name).

When fork() (the underlying system call) is called, the process is split into 2 identical copies, and the fork call returns in each copy, indicating by its return value which copy is which. (That's a simplified view, but it'll do for now)

So your concatentate_parallel function is effectively adding a letter to $content in each subprocess, but that doesn't affect the parent process (your main application) where $content remains unchanged.

You can find a lot of good info online about how fork works, what the variants are, what gets copied for subprocesses and what doesn't. It's a good thing to understand.

By the way, if you'd been using threads instead, you'd have been on the right path. Threads are simultaneous paths of execution in the _same_ process, so different threads could modify $content. But without using synchronization mechanisms, you're right, the letters would arrive in a jumbled, unpredictable order.

Good luck!