#!/usr/bin/perl

use strict;
use warnings;

my $previous;

&unique while(<DATA>);

sub unique
{
  return if $previous and $previous eq $_;
  print;
  $previous = $_;
}

__DATA__
a1a
a1a
b1b
c1c
c1c
d1d
d1d
e1e
f1f
g1g
g1g
h1h
h1h
i1i
j1j
k1k
k1k
k1k