When using your example of 

$ftp_user = 'DOMAIN\\user1'; 

I get the error:

Could not login to 9.9.9.9 1000 with user DOMAIN\user1:  at ./test.pl line 34.  ##1000 is the port

sed -n 34p test.pl showed me :

$ftp->login($ftp_user, $ftp_password)  #line 34