A 58-year-old man presents to the emergency department with acute onset severe epigastric pain and vomiting for 6 hours. He has a history of peptic ulcer disease and stopped his proton pump inhibitor 3 months ago. On examination, he is febrile (38.2°C), tachycardic (110 bpm), and has severe abdominal rigidity with guarding. Upright chest X-ray shows a large pneumoperitoneum. CT abdomen with oral and IV contrast confirms free air in the peritoneal cavity with a focal defect in the anterior wall of the gastric antrum. What is the most likely diagnosis?