-- run as system
CREATE DIRECTORY UTL_DIR AS <dir in which XML file is placed>;
GRANT READ ON DIRECTORY UTL_DIR TO <dbuser>; 

-- run as <dbuser>
DECLARE
  v_file      utl_file.file_type;
  v_dir       VARCHAR2(120) := 'UTL_DIR';
  v_filename  VARCHAR2(100) := 'datafile.xml';
  c           CLOB := NULL;
  x           XMLTYPE;
  str         VARCHAR2(1000);
BEGIN
  -- Read the file into CLOB variable
  v_file := utl_file.fopen('UTL_DIR', v_filename, 'r');
  WHILE (TRUE) LOOP
    BEGIN
    utl_file.get_line(v_file, str);
    EXCEPTION
      WHEN NO_DATA_FOUND THEN
        utl_file.fclose(v_file);
        EXIT;
    END;
    c := c||str;
  END LOOP;

  -- Cast CLOB data as XMLTYPE instance
  x  := XMLTYPE(c);

  -- Process the XMLTYPE instance
  -- ...

END;
/