I think you're looking for something along the lines of the following:

use Net::FTP;

$ftp = Net::FTP->new($host, Debug => 0)
             or die "Cannot connect to $host $@";

$ftp->login($user_name,$password)
             or die "Cannot login ", $ftp->message;

$ftp->cwd($target_dir)
             or die "Cannot cd to $target_dir", 
                    $ftp->message;
my @files = $ftp->ls('nok*');

for (@files) {
   $ftp->get($_)
             or die "couldn't get $_", $ftp->message;
   ... do something with the file ...
}

$ftp->quit;
[download]

Check the response from the ls command when you debug; for some hosts you may need to rework the ls output to get it into the right format.