From The Collaborative International Dictionary of English v.0.48 :
Perforce \Per*force"\, adv. [F. par (L. per) + force.]
By force; of necessity; at any rate. --Shak.
[1913 Webster]
From The Collaborative International Dictionary of English v.0.48 :
Perforce \Per*force"\, v. t.
To force; to compel. [Obs.]
[1913 Webster]
From WordNet (r) 3.0 (2006) :
perforce
adv 1: by necessity; by force of circumstance