#!/usr/bin/perl
use strict;
use warnings;

my $derp = 'X' x 40000;
open (my $fh, '>', 'derp.txt') or die "can't open derp.txt: $!";
print $fh $derp;
close($fh);

##</code><code>##

ls -al derp.txt 
-rw------- 1 marto marto 40000 Sep 21 09:37 derp.txt