my %seen = ();
my $tmp

...
$seen{$_}++;
if ($_ ne $tmp && $seen{$tmp} == 1){
  print $_;
  $tmp =$_;
}elsif ($_ eq $tmp){
  print $_ if $seen{$_} == 2;
}


##</code><code>##


my %seen = ();
my $tmp;
my $id;
...
if ($_ ne $tmp){
$seen{$tmp}=$id++;
}
and then in the second loop:


print $_ foreach (sort{$seen{$a}<=>$seen{$b}}keys %seen);