#!/usr/bin/perl -l
use strict; use warnings;
$_ = do{local $/; <DATA>};
/^((\w*) [rs] (\w*))$ # find a first word
  .* # skip non match lines
   ^(\2 [rs] \3)$ # second word same except rs
  (??{print "$1 $4"})/xms; # print and fail
__DATA__
bare
mare
base
case
bust
burt
bent
sat
rat
bat
matter
mattes
pat