#!/usr/bin/perl -w
use strict;

$\ = "\n";
my %h;

open FA, "file_a.txt"; 
for(<FA>){chomp; $h{$_} |= 1}; 
close FA;

open FB, "file_b.txt"; 
for(<FB>){chomp; $h{$_} |= 2}; 
close FB;

my @keys = sort keys %h;

print"Only File a:"; 
for (@keys) {print if $h{$_} == 1}; 

print "\nOnly in File b:"; 
for (@keys) {print if $h{$_} == 2};

print "\nIn both files:";
for(@keys){print if $h{$_} == 3};