# Simple enough?!?
sub remove_duplicate_lines {
    my ($infile, $outfile)=@_;

    open my $in,  '<', $infile  or die "$infile: $!\n";
    open my $out, '>', $outfile or die "$outfile: $!\n";
    my $oldout=select $out;

    my %saw;
    $saw{$_}++ or print while <$in>;
    select $oldout;
}