There isn't a direct way to do this. You can use p4 print, as someone else already suggested, or p4 sizes, which gives you the the size of a file revision in bytes (and you'd have to calculate the difference between subsequent revisions).
I recommend paying special attention to integrations. If your Perforce server employs branches, you'll probably want to discount integrations from your metrics calculation. Only the original change should get credit for adding/modifying lines, pushing that change to another codeline should not.