"Light and agile" eh? 

#!/usr/bin/perl -l
use strict; use warnings;

$_ = do{local $/; <DATA>};

/^((\w*) (.) (\w*))$ # find a first word
  .* # skip non match lines
   ^(\2 (?!\3). \4)$ # second word same except one letter
  (??{print "$1 $5"})/xms; # print and fail

__DATA__
cases
carer
caser
cares
bare
mare
base
case
bust
burt
bent
sat
rat
bat
matter
mattes
pat

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In reply to Re^2: Comparing Lines within a Word List by Anonymous Monk
in thread Comparing Lines within a Word List by dominick_t

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