You could fork or provide system() arguments which will put each submitted command into the background, allowing the next iteration to begin immediately. However, this will mean that you're creating many concurrent login sessions to the same FTP server, which may fail if the FTP server has policies against such usage. You may also want to limit the number of concurrent sessions for other reasons, even if "bandwidth isn't a problem."

my %children;
my $max_children = 3;
my @todo_list;


foreach my $dir(qw/lmo lin lmf bsu ssu sst/)
{
    fork_download($dir);
}
complete_downloads();


sub fork_download
{
    my $dir = shift;

    if ( ( scalar keys %children ) < $max_children )
    {
        my $pid = fork;
        if ( $pid )
        {
            #   this is parent process
            $children{$pid} = $dir;
        }
        else
        {
            if ( not defined $pid )
            {
                die "failed to fork!\n";
            }
            #   this is child process
            my $link = "ftp://ftp.<Path_to_dir>.jp/$dir/*";
            system("wget -nH -nd  --timestamping $link");

            #   exit child process
            exit 0;
        }
    }
    else
    {
        #   too much child labor! queue for later
        push @todo_list, $dir;
    }
}

sub complete_downloads
{

    while (%children)
    {
        my $childpid = wait();
        print "child [$childpid][" . $children{$childpid}. "] is done\
+n";
        delete $$children{$childpid};
        
        if ( @todo_list )
        {
            my $nextDir= shift @todo_list;
            fork_download($nextDir);
        }
    }
}
[download]