The Collaborative International Dictionary of English v.0.48:

Perk \Perk\ (p[~e]rk), v. t. [imp. & p. p. Perked (p[~e]rkt);
   p. pr. & vb. n. Perking.] [Cf. W. percu to trim, to make
   smart.]
   To make trim or smart; to straighten up; to erect; to make a
   jaunty or saucy display of; as, to perk the ears; to perk up
   one's head. --Cowper. --Sherburne.
   [1913 Webster]

   to perk up one's ears to begin to listen attentively; --
      usually used when something of interest is mentioned in
      some discourse, after one's attention had lapsed; as, he
      perked up his ears when he heard his name mentioned.
      [PJC]