I couldn't get your script to work, so I followed salva's advice. This worked:

#!/usr/bin/perl

use strict;
use warnings;
use diagnostics;
use Net::SFTP::Foreign::Compat ':supplant';

my %args = (password => 'yourpassword', user => 'yourusername');

warn "Starting SFTP...\n";
my $sftp = Net::SFTP::Foreign::Compat->new('localhost', %args);
print 'done', "\n";

warn "Starting Command...\n";
$sftp->get('/root/Desktop/pdf.pl', '/home/Desktop/tmp1/pdf.pl');
print 'done', "\n";

undef $sftp;
[download]