Starting with 5.8.1, Perl began caching ppid values in an attempt to hide the fact that Linux assigns different process ids to different threads in the same application, it was thought that getppid in perl should behave the same across platforms so this caching was introduced. There was a lot of argument over whether this was a good thing or not. I'm not sure when it was changed back (or fixed), but I can say that 5.8.6 behaves as you would expect. It's also possible that this behavior could be different between the same versions of perl, depending on whether you compiled with thread support or not.