Wal-Mart Stores Inc. owns more than $76 billion of assets through a web of units in offshore tax havens around the world, though you wouldn’t know it from reading the giant retailer’s annual report. A new study has found Wal-Mart has at least 78 offshore subsidiaries and branches, more than 30 created since 2009 and none mentioned in U.S. securities filings. Overseas operations have helped the company cut more than $3.5 billion off its income tax bills in the past six years, its annual reports show.