inline_file_sort.pl

use strict;
use warnings 'all';
use Tie::File;
use Fcntl 'O_RDWR';

my $file = "to-sort.txt";
tie my @nums, 'Tie::File', $file
  , mode => O_RDWR
  , recsep => ', '
  , memory => 20_000_000 # max 20 MB read buffer
  , autodefer => 0 # flush every write
  ;

mysort();

sub mysort {
  return if (scalar @nums <= 1);
  chomp $nums[-1];
  TOP: for (my $i=1; $i < scalar @nums; $i++) {
    if (mycmp($nums[$i-1], $nums[$i]) > 0) {
      my ($num) = splice @nums, $i, 1; # remove the number
      # find correct position
      for (my $j=0; $j < $i; $j++) {
        if (mycmp($num, $nums[$j]) <= 0) {
          # found it, put the number here
          splice @nums, $j, 0, $num;
          next TOP;
        }
      }
    }
  }
}

sub mycmp {
  #return rand() - 1/2;
  return $_[0] <=> $_[1];
}