Perpessicius joined the Romanian Army in 1916, as Romania rallied with the Entente against the Central Powers ("see Romania during World War I"). He was sent to Northern Dobruja in the wake of the Turtucaia defeat, when southern Romania was being invaded by Bulgarian and Imperial German forces. On October 6, 1916, during the skirmish of Muratan, his right elbow was hit by an enemy bullet, being partially amputated by surgeons in order to prevent a loss of the entire limb. His disabled arm was fastened with a black-colored sling, and Perpessicius taught himself to write left-handed (a change which reportedly made his handwriting instantly recognizable by his peers).