Usually you get this error if you forget to 'use' the module, or your module installation is corrupted, and Net/FTP.pm might actually be an empty file or somesuch. For example, running just that line of code without the use line gives me the error. However, this code works:

use Net::FTP;

$ftp=Net::FTP->new("$hostname");
[download]

Maybe you could post some more code, especially the 'use' line that you're, well, using.