Just insert a last line after you last action in the code.
The action with the "//uses: ;" above it must not end on the last line. Dunno why. (at least in A5)
But just edit your code to this:
Code:

---

// use: _base_length_x, _base_length_y
action spawn_square_plane
{
	temp_creation = square_plane();
	spawn(temp_creation);
} 

//END OF SCRIPT

---

And it should work.