There's no need for a script at all. VX has Areas. If you check out the mapping editor, you'll notice that you can make Maps and Areas. Areas are rectangular zones that you create on maps that can have a different set of monsters than what you set for the Map it's on. So say you set your map to only allow Slimes to attack you every 30 steps. You can set an Area on that map to make Goblins attack you every 30 steps as well. If you're on the map (not the area) then approximately every 30 step, you'll fight a Slime. If your in the Area on the map, then every 30 steps you'll fight either a Slime OR a Goblin.
If you want each Area to be unique, then do NOT set any monsters to attack on your map. Set ALL of the monsters you want to have in the Areas alone and make sure you cover your whole map with different Areas.
What YOU need to do, is make a World Map and then in the desert zone, create an Area and make that Area's monsters different than the Area you will set for a snowy zone. Just remember that you can only make rectangular Areas so if you have oddly shaped zones, you'll need to make more than 1 Area to fit the zone.
Also, a world map is nothing but a normal map but with different autotiles. Look up how to make a world map or look at the VX Resource Database for "TileE 3" for my TileE that has unique world map tiles as well as other extensions for you game graphics.
Moral of the story ~ you don't need a script in the least. VX does this already.