use strict;
use warnings;

my $prev;

while (<DATA>) {
    chomp;

# Note that $prev will be undef only on the first time round.

    my $curr = $_;
    if (!defined $prev)
    {
        $prev = $curr;
        next;
    }

    if ($curr ne $prev) {
        print "$prev\n";
        $prev = $curr;
    }
    else
    {
        undef $prev;
    }
}

print "$prev\n" if defined $prev;

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