use strict;
use warnings;
use File::Slurp qw/read_file/;

my ( %file1Hash, %file2Hash, %mergedHash );
my @files = sort { -M $a <=> -M $b } <"*.txt">;

do { chomp; $file1Hash{$_}++ }
  for read_file $files[0];
$mergedHash{$_}++ for keys %file1Hash;

do { chomp; $file2Hash{$_}++ }
  for read_file $files[1];
$mergedHash{$_}++ for keys %file2Hash;

print "$_\n" for grep $mergedHash{$_} == 1, keys %mergedHash;