ActiveState Perl installs an IIS script mapping for the extension .plx. Is this actually used in real life or just something specific to ActiveState?
No, it's not just from ActiveState. O'Reilly's Learning Perl on Win32 systems recommends naming scripts with a .plx extension to disambiguate them from perl modules (with .pm) and non-executable perl libraries (.pl). Nowadays however I'd name anything that is going to be directly run as .pl.