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]

-HTH

Update: forgot to exit child process!