Here's one in 24 characters (not counting shebang):

#!/usr/bin/perl -wl
print eval"@ARGV[0,2,1]"
[download]

Update: ARGG! I started along the lines of japhy's solution and suddenly, for no apparent reason, my brain skipped a neuron or ten and decided that one postfix op was the entire problem instead of an arbitrary rpn expression. Move along now ... no need to slow down and stare at the crash victim ...