Do you have control over the application generating the files?

Unfortunately no. See my reply to Perrin below.

What happens if the parent process the directory as files are created and spawns the child to process before the file is actually closed and static...

Files placed in the directory are zero byte size until the legacy app closes them (i.e completes the write). I should have included this line of code in my original example. The files themselves are tiny, less than 200 bytes.

next if (! -s $fullPath);

I assume you handle duplicate file names in the fullpath/newpath locations well?

The legacy app creates the file with the date/time/PID in the filename. The timestamp goes down to milliseconds so that combined with the PID of the app should ensure a unique filename.