Here, I found the following that seems to be included in the module docs:

When a non-dependant PAR package runs, there is a two stage binary frontend: first the executable demo.exe that I ran, then a second demo.exe that is extracted to the temp dir and run as a child of the first. If you look at the process list while demo.exe runs, you'll see two processes with the same name.

I'm guessing the first process is the one that extracts the perl binary and your sources from the exe, then calls the extracted binary to run the extracted sources as a separate process (to handle crashes better maybe?), and then cleans up the extracted files when perl exits.

At least I seem to recall someone once mentioning some article where someone else said that's how it might more or less work. (In other words, I'm guessing)