I performed another experiment: I copied the PerlTk script from our work environment (EL8+GNOME3) to my home PC (Fedora 41 + KDE). After installing some needed Perl modules and bringing the icon JPG file and fixing the path of the icon file the script started running and the icon was correct. So it seems the problem wasn't with the script buy with the graphic environment of the OS. And how to debug this - I have no idea...