Perini Building, which had a $6.29 billion construction contract with MGM Mirage's CityCenter in Las Vegas, has appealed to Nevada Gov. Jim Gibbons for help on $500 million in claims the builder says it is still owed. Perini filed a master lien against the project for that amount on March 29. Payment to about 600 subcontractors and vendors hangs in the balance. MGM Mirage is reviewing Perini's letter.