Thanks! It appears (through some experiments on several different versions of Windows - sorry for the delay) that having the extension 'lnk' is sufficient proof that a file is indeed a shortcut file. I had thought of that but assumed (foolishly it seems) that one could re-name a shortcut without destroying the shortcutting capability. It seems that this is not the case - you can't rename a shortcut to eliminate the '.lnk' (short of editing the directory I imagine).
However: if you rename any file to add '.lnk', the file seems to be permanently changed (since you can't remove the '.lnk') to a shortcut, though it's content is retained and can still be manipulated. In this case, the shortcut properties are inaccessible which makes me think that it isn't really a shortcut after all. Now, if you use Win32::Shortcut's Save() on such a file that isn't really a shortcut, its contents are then destroyed and replaced with the guts of a standard .lnk file and it becomes a real shortcut. Just to be weirder, I tried Save() on a normal file without '.lnk', and it too had it's content replaced, but the Explorer didn't recognise it as a shortcut as the extension was not 'lnk'. I think I'll stop now!
It would be better if it were simply impossible to add '.lnk' to an existing normal file.
I'd like to be able to assign to an luser