Thanks for your suggestion, I used 5.12.0 for a long time and was not aware that JSON::PP was incorporated in 5.13.
While I mention 5.22 in my tests, one of the targeted platforms, where I have no possibility to use non-core packages has a system Perl of 5.16.3. So it could work. (Of course I could cheat with carton, but as the script could be run if needed on any of thousands of hosts without being installed at first it becomes teddious to deploy some local dependencies prior to run the script.)
BUT: my initial goal is to understand the cause of the unexpected behavior. ;o)
The best programs are the ones written when the programmer is supposed to be working on something else. - Melinda Varian