A neonate presented with fever, lethargy, abdominal distension, vomiting and constipation. Clinically he was diagnosed as volvulus neonatarum with suspected perforation. The best investigation would be –
Volvulus neonatorum 

The fetal alimentary canal returns from the extraembryonic coelom into the abdomen at 8-10 weeks and the bowel undergoes rotation and fixation at certain points by the attachment of its mesentery to the posterior abdominal wall.
When the process is incomplete or deviates from the normal plan, the result is malfixation or malrotation.
Commonly, the normal oblique attachment of the mesentery from duodenojejunal flexure of the caecum is absent, and the small bowel is attached to the posterior abdominal wall by a narrow stalk based around superior mesenteric vessels. The caecum is undescended, i.e., situated in the right hypochondrium and abnormally fixed by peritoneal bands running laterally across the second part of the duodenum.
The poorly attached small bowel undergoes volvulus around the axis of the 'universal mesentery', which is twisted so that the flow of blood is cut off, producing a strangulating obstruction of the small bowel. This typically occurs in the newborn, hence the term `Volvulus neonatorum'. The terminal ileum and caecum are drawn into the volvulus and are wrapped around the stalk of the mesentery in two or three tight coils.

Clinical feature 

A young infant, previously healthy, starts bile-stained vomiting during the first few days of the live. The abdomen is soft and non-distended; however, abdominal distension may occur which is limited to upper abdomen (epigastrium).
No obstruction may occur in the first day or two after birth and meconium may be passed normally. Then, with variable suddenness, bowel actions cease with onset of obstruction.
The signs vary, depending on the degree of intestinal obstruction versus ischemia. When strangulation occurs there are signs of shock, especially pallor and a vague mass of congested bowel may be palpable in the centre of the abdomen. Blood or blood-tinged mucus may be passed rectally.
Ischemia may result in gangrene and perforation.

Investigations  

Barium contrast studies with fluoroscopy (Barium meal follow through) is the investigation of choice as X-ray is not reliable in early stages of volvulus.
However, if the perforation is suspected, barium contrast should not be used because of the risk of peritonitis. Water-soluble contrast studies (iodine based) should be used. Plain X-ray of abdomen/chest is the initial investigation to demonstrate free air under the diaphragm in suspected cases of perforation.
Plain x–ray
Barium enema
Upper GI endoscopy
Barium meal follow through