To follow zentara's advice, try this:

#!/usr/bin/perl

use strict;
use warnings;
use Net::SSH2;
use Net::SFTP::Foreign;
use Net::SFTP::Foreign::Backend::Net_SSH2;

my $host = 'localhost';
my $user = 'user';
my $pass = 'pass';

my $ssh2 = Net::SSH2->new();
$ssh2->debug(1);
$ssh2->connect($host)
  or die "Connect failed!\n";
$ssh2->auth_password($user, $pass) 
  or die "password auth failed\n";
  
my $sftp = Net::SFTP::Foreign->new(ssh2 => $ssh2,
                               backend  => 'Net_SSH2');
$sftp->error and
  die "Unable to establish a SFTP connection: ",
  $sftp->error;
$sftp->put('/null.txt', '/root/Desktop/null.txt');
[download]