The same way any other infix operator would, the operator has to have an expression on either side so the first % can't be an operator, and you can't have two expressions next to each other anyway i.e. you can't have "x x" or "f(x) x" or "% %"., so the second % has to be the % operator.
Definitely looks strange and could be confusing at first, but the syntax is unambiguous to parse. Add a comment, write "|> (%) % 2" perhaps, or just create a mod function and write "|> mod(%, 2)".
Definitely looks strange and could be confusing at first, but the syntax is unambiguous to parse. Add a comment, write "|> (%) % 2" perhaps, or just create a mod function and write "|> mod(%, 2)".