{
	my %seen;
	my ($in_file, $out_file) = qw| in.txt out.txt|;
	open my $in_fh, $in_file or die "$in_file: $!";
	open my $out_fh, '>', $out_file or die "$in_file: $!";
	while ( <$in_fh> ) {
		print $out_fh $_ unless $seen{$_}++;
	}
}