I believe that this line is what's slowing you down.

You are wrong to believe that. I suggested this nearly a month ago, and the OP tested it and verified that avoiding the concatenation did not make a difference.

Since then, the OP advised us that they had raised it as a perl issue on github; it has been investigated there, and tonyc++ just reported back to us on progress made towards fixing the problem.

For example, I noticed that sometimes in Windows when a scalar goes out of scope, it should be cleaned up. Right? But that's not what happens. Instead, it stays in the memory until the perl script ends. Or if you say "undef $Variable" then it cleans up the memory and I can see a drop in memory usage when I'm watching Task Manager. But otherwise, it just stays there like a zombie

This is a behavior of old perls, such as 5.8. The "my" variables get allocated and then under the assumption that the function will get called again with similar-sized scalars, they don't get freed, just overwritten on the next function call. Newer perls have some kind of heuristic to prevent this from gobbling large amounts of memory.