A 58-year-old man from Delhi presents to the emergency department with acute onset severe epigastric pain radiating to the back, accompanied by vomiting. He has a history of peptic ulcer disease and stopped taking his proton pump inhibitor 3 months ago. On examination, he is febrile (38.2°C), tachycardic (HR 110/min), and has severe tenderness in the epigastrium with guarding. Upright chest X-ray shows pneumoperitoneum. Which imaging finding on CT abdomen would be most specific for identifying the site of perforation in this clinical scenario?