OK. Your point is accepted and verified by me on Perl v5.24.

I actually saw this in my testing, but I couldn't believe my own eyes!
The exact same source code can produce different results when run more than once.

I am flabbergasted! This is true, but I don't know why or how?

More Info:
Updating a Windows GUI is an extremely expensive operation.
If you have a loop that will do say 10,000 operations, and you want to show progress via a percent or a progress bar, calculate whether or not the progress bar will move before you update the GUI's value - do this at most 100 times (each one percent for a 100% bar) and probably less than that! The difference in performance between calling the GUI 10,000 times vs 100 times is astonishing!