With its cavernous cargo holds and gleaming white superstructure, the 730-foot Perla del Caribe, which is nearing completion at a shipyard here, looks like any midsize container ship. But at the stern, visible through cutouts in the superstructure, are two huge steel tanks that show this is no typical vessel. The bulbous tanks, 90 feet long and painted an eye-catching lime green, together will carry half a million gallons of liquefied natural gas, or L.N.G.,…