#!/usr/bin/env perl

use strict;
use warnings;

my $COUNT = $ARGV[0];
my %seen;
my @data;

unshift @data, split while <DATA>;
@data = grep !$seen{$_}++, @data;
print "@data[0 .. $COUNT - 1]\n";

__DATA__
1 1 2 2 3 3 4
5 6 7 8
5 6 6 7 8
5 6 7 8
9 10 11 12
13 14 15 16
17 18 19 20
17 18 19 20
21 22 23 24

##</code><code>##

$ ./pm_11109149_reverse_file_extract_with_conds.pl 24
21 22 23 24 17 18 19 20 13 14 15 16 9 10 11 12 5 6 7 8 1 2 3 4

$ ./pm_11109149_reverse_file_extract_with_conds.pl 12
21 22 23 24 17 18 19 20 13 14 15 16

$ ./pm_11109149_reverse_file_extract_with_conds.pl 7
21 22 23 24 17 18 19