Symbolism, Imagery, Allegory
It's not just once that Perry and his fellow soldiers refer to their homes in America as the World. World, with a capital W, like it's a proper noun. Making it seem like a bigger deal, and much farther away.
For example. "I had never been a coffee drinker in the World, but now I wanted it. Now I needed it, anything that promised to get me to the next minute." (20.13)
Why not just say "back home?" And why the previous aversion to coffee?
Referring to "The World" is more about where they are now—a place that they see as not being in the world, or at least the world they're familiar with. Vietnam is, of course, on Earth (just to clear that up).
But the wartime zone they exist in seems so wrong and so inhumane to them that it might as well be a terrible otherworldly place, like Hell. (And you know Myers is always using supernatural imagery by referring to dead soldiers as angels, so it makes sense that he'd bring Hell into the picture.)
And it does sound pretty hellish. This war is a place where babies are killed, where children are mined, where nothing, not even the innocence of childhood, is sacred. It's a place where sometimes American soldiers kill other American soldiers by accident, and where Perry has to be okay with killing the enemy, even if he sees himself in some of the young men in the Vietcong.
To Perry and his fellow soldiers, the World is like a faraway dream. It's the goal, but sometimes, when things get really bad, they can't imagine going back.