'perception of impermanence', is defined in the Girimananda Sutta (A.X. 60) as meditation on the impermanence of the five groups of existence.
"Though, with a faithful heart, one takes refuge in the Buddha, his Teaching and the Community of Monks; or with a faithful heart observes the rules of morality, or develops a mind full of loving-kindness, far more meritorious it is if one cultivates the perception of impermanence, be it only for a moment" (A.X. 20).
See A.VI. 102; A.VII. 48; Ud.IV. 1; S.22. 102.