SELECT to_char(val), val - val FROM (
  SELECT utl_raw.cast_to_number('ff65') val FROM dual
)

TO_CHAR(VAL)                                VAL-VAL
---------------------------------------- ----------
~                                                 0

    dbms_output.put_line(x + 2 * y);

    x := x + 2 * y;

    dbms_output.put_line(x);
    
    dbms_output.put_line(utl_raw.cast_from_number(x));
  end;

DECLARE
  x NUMBER;
  f binary_double;
BEGIN
  f := BINARY_DOUBLE_NAN;

  IF f IS NAN THEN
    dbms_output.put_line('ook');
  END IF;


  BEGIN
    x := f; -- ORA-06502 is here

    IF x IS NAN THEN
      dbms_output.put_line('oops');
    END IF;
  EXCEPTION
    WHEN OTHERS THEN dbms_output.put_line(SQLERRM);
  END;

  x := utl_raw.cast_to_number('FF65');


  IF x IS INFINITE THEN
    dbms_output.put_line('ook');
  END IF;

  f := x;

  IF f IS INFINITE THEN
    dbms_output.put_line('ook');
  END IF;
END;
/

ook
ORA-06502: PL/SQL: numeric or value error
ook
ook

SQL> var n number
SQL> begin
  2   :n:=
  3    9.999999999999999999999999999999999999999e125
  4   +0.000000000000000000000000000000000000001e125;
  5  end;
  6  /

PL/SQL procedure successfully completed.

SQL> print n;

         N
----------
         ~

SQL> select
  2    9.999999999999999999999999999999999999999e125
  3   +0.000000000000000000000000000000000000001e125
  4  from dual
  5  /
 +0.000000000000000000000000000000000000001e125
 *
ERROR at line 3:
ORA-01426: numeric overflow

SELECT CAST('+INF' AS binary_float)  ||'' FROM dual;
SELECT cast(CAST('+INF' AS binary_float) AS NUMBER) ||'' FROM dual; --oops is here
SELECT CAST(utl_raw.cast_to_number('FF65') AS binary_float) ||'' FROM dual;