#!/usr/bin/perl

use strict;
use warnings;
use Getopt::Std;

our ( $opt_i, $opt_o );
my ( $ifh, $ofh );

getopts( 'i:o:' ) and $opt_i and $opt_o 
    or die "Usage:  $0  -i infile -o outfile\n";
warn "reading \"$opt_i\" and writing to \"$opt_o\"\n";

open( $ifh, "<", $opt_i ) or die "$opt_i: $!\n";
open( $ofh, ">", $opt_o ) or die "$opt_o: $!\n";
binmode $ifh;
binmode $ofh;

my %lines;
$lines{$_}++ while (<$ifh>);

my $line_count = 0;
for (sort keys %lines) {
    print $ofh $_;
    $line_count += $lines{$_};
}
close $ofh or die "error on closing output file: $!\n";

warn "read $line_count lines from $opt_i, wrote ".scalar(keys %lines).
    " lines to $opt_o\n";