Since Perennius had no idea of what was happening, he was caught totally unawares when Commodus sent someone by night to behead him.
Edition :
Herodian. History of the Empire, Volume I: Books 1-4. Translated by C. R. Whittaker. Loeb Classical Library 454. Cambridge, MA: Harvard University Press, 1969.
Herodian. History of the Empire, Volume II: Books 5-8. Translated by C. R. Whittaker. Loeb Classical Library 455. Cambridge, MA: Harvard University Press, 1970.
Notes :
The son of Perennis is also killed while returning to Italy, described in: "One of the Sons of the praetorian prefect Perennis is killed on orders of Commodus."