SQL> set serveroutput on
SQL> declare
  2    x0 int := 3;
  3    y0 int := 5;
  4  procedure swap(x number, y number) as begin y0 := x; x0 := y; end;
  5  begin
  6    dbms_output.put_line('x0 = ' || x0 || ' y0 = ' || y0);
  7    swap(x0 + 0, y0 + 0);
  8    dbms_output.put_line('x0 = ' || x0 || ' y0 = ' || y0);
  9  end;
 10  /
x0 = 3 y0 = 5
x0 = 5 y0 = 3

PL/SQL procedure successfully completed.

SQL> declare
  2    x0 int := 3;
  3    y0 int := 5;
  4  procedure swap(x number, y number) as begin y0 := x; x0 := y; end;
  5  begin
  6    dbms_output.put_line('x0 = ' || x0 || ' y0 = ' || y0);
  7    swap(x0, y0);
  8    dbms_output.put_line('x0 = ' || x0 || ' y0 = ' || y0);
  9  end;
 10  /
x0 = 3 y0 = 5
x0 = 3 y0 = 3

PL/SQL procedure successfully completed.

alter session set plsql_optimize_level = 0;