#!/usr/bin/perl -w
use strict;
use vars qw ($outfile @array);

open(DATA, $ARGV[0]) or die "Couldn't open $ARGV[0]: $!";
$outfile = "whatever.txt";
open (OUT, ">$outfile") || die "Can't open $outfile for creation: $!\n";

##############
##############  This section could be used if one doesn't care about order
#my %uniq;
#$uniq{$_}++ while <DATA>;
#print OUT for keys %uniq;
##############
##############

my (@array, %hash);
while (<DATA>) {
    push (@array, $_) unless (defined($hash{$_}));
    $hash{$_} = 1;
}

print OUT join("", @array);