1939 Perla and performed a cruise in the Indian Ocean to test the performance of the class in terms of sailing and operational capability in warm seas and during the monsoon season. The results were disappointing: it was discovered that stormy sea prevented the use of submarine's armament and made it very difficult even to remain at a periscope depth. There were also reports of malfunctioning air conditioning system, and leaks of chloromethane were reported. In 1939 she temporarily returned to Italy for repairs and maintenance. In the summer of 1939 she was involved in testing the effectiveness of a "bubble-free" torpedo system which occurred in Naples. The existing system on Italian submarines released large amounts of air bubbles during torpedo launch, which helped in spotting the torpedoes and the location of the submarine. The tests passed with satisfactory results. At the time of Italy's entrance into World War II Perla was again in Massawa as part of 82nd Squadron and commanded by captain Mario Pouchain. According to the initial orders, Perla was supposed to remain in Massawa during the first weeks of the war, but Viceroy Amedeo de Aosta demanded that two additional submarines be deployed in the operations area. Rear Admiral Carlo Balsamo, Massawa Base Commander, agreed to the request and sent out Perla and . Perla left Massawa on the afternoon of 19 June 1940 to patrol an area in the Gulf of Tadjoura about 15 nautical miles from Ras-El Bir. Perla was supposed to remain at her station until 9 July and attack any enemy ships she came across. On 20 June an electrician suffered a heat stroke due to the malfunctioning air-conditioning system. The captain ordered inspection and maintenance to be conducted, and the submarine surfaced during the night of 21 June to minimize the risk of poisoning. At dawn, Perla submerged and soon after several crew members fell ill. As time went by, more men became ill, and the ones already suffering started exhibiting signs of chloromethane poisoning, including hallucinations. On 22–23 June Perla patrolled her designated area. On 24 June another submarine returned to Massawa with half of her crew poisoned by chloromethane, and Perla was ordered to return to Massawa immediately to undergo repairs. Perla turned around during the night of 24 June, but the next day the commander fell sick too, and more than half of the crew was suffering from