this is actually possible, or at least used to be possible in older windows, even though "normally" the same environment forking model applied. it involved a routine in assembly language. i have actually compiled and ran it myself, and it worked. the routine accessed the system environment variable and changed it. this is not something perl can do. probably can't be done that easy now, now that operating systems have become so much tighter in controlling processes. it is possible though, as that's exactly what gets done when you adjust the enironment variables in the tab for these settings (right click "my computer", select "advanced" tab, and click "environment variables".
years ago you could also run a compiled program and just poke around in memory into other programs etc. no can do now. now we have a perfectly stable OS thanks to Billy, for five minutes at least ;)