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 treated intermittently. On examination, he is febrile (38.5°C), tachycardic (110/min), and has severe epigastric tenderness with guarding. An upright chest X-ray is obtained. What is the most characteristic radiological finding that would support the diagnosis of perforated peptic ulcer?