#!/usr/bin/env perl

use warnings;
use strict;
use Sort::External;
use Unicode::Collate::Locale;

my $in  = shift // 'large-unsorted.txt';
my $out = shift // 'sorted.txt';

my $comparator = Unicode::Collate::Locale->new(locale =>'vi');

my $sorter = Sort::External->new
(
  sortsub => sub { $comparator->cmp($Sort::External::a, $Sort::External::b) }
);

open my $unsorted, '<', $in or die $!;

$sorter->feed($_) while <$unsorted>;
$sorter->finish(outfile => $out);