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

my %wanted;

while (<DATA>) {
    exists $wanted{$_} ? delete $wanted{$_} : $wanted{$_}++;
}

print sort keys %wanted;

__DATA__
a1a
a1a
b1b
c1c
c1c
d1d
d1d
e1e
f1f
g1g
g1g
h1h
h1h
i1i
j1j

##</code><code>##

b1b
e1e
f1f
i1i
j1j

##</code><code>##

 perl -ne 'exists $x{$_}?delete $x{$_}:$x{$_}++;}{print for sort keys %x;' < input.txt > output.txt