A region is not an object; an object is a hero, a castle, or, say, a bush with a name. The function receives the name of the hero who entered the region, and it's easiest to use that. Player_1 probably won't work because it's not the same as PLAYER_1.
I would recommend downloading any story-driven map that is rich in scripts and simply searching for an example of how to use any unclear function within it.
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