The documentation for perpendicular coordinates says

horizontal: The coordinate through which the horizontal line passes.
vertical: The coordinate through which the vertical line passes.

You can use the implicit syntax of (horizontal, "-|", vertical) or (vertical, "|-", horizontal).

while the actual behavior of the keywords seems like the other way round. Look at the example in the documentation:

line("p1.south-east", (horizontal: (), vertical: "xline.end"))
line("p2.south-east", ((), "|-", "xline.end")) // Short form

In the first line it's actually the horizointal line that goes through `xline.end``.

The short form looks right: - indicates a horizontal line going through xline.end.