The print arrow is very commonly used to show what is on the stack. It has two different uses, one inside procedures and one at "top level" i.e. when you are giving individual commands to Pop-11.
1. In a procedure "=>" prints only one item, the one on top of the
stack, and removes it from the stack.
2. At "top level" "=>" prints out everything on the stack, bottom
item first, top item last, and clears the stack.
The reason for the restriction in (1) is that when procedures are
running there may be several items on the stack waiting to be dealt
with by other procedures than the one that calls =>. So those items
should not be removed. (This restriction can be overcome by using the
popval procedure to simulate top level instructions, in the format:
popval( [=>] );
But that's recommended only for experienced programmers.)