Migrate database through database link
The USING clause expects a valid TNS entry or alias. Use this:

CREATE DATABASE LINK OLD_FOO
CONNECT TO FOO IDENTIFIED BY "pa$$w0rD"
USING '(DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = old.example.com)(PORT = 1521)) (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = XE)))';