Yes I know :-) and I am looking for the way to achieve it. So far I coded like below:

my @childs;
my @allfolders  => List of all folders

for ( my $i=0; $i<@allFolders; $i+=5 ) {

    my @sourceFolders = @allFolders[$i..$i+4];


        foreach my $folders ( @sourceFolders ) {

            my $pid = fork();

            if ( $pid ) {

                # parent
                push(@childs, $pid);
        
             } elsif ($pid == 0) {

        EXECUTe RSYNC HERE....
    
            } else {
                print "Error: Cannot fork process : $! \n",1);
            }
        }

        
    foreach ( @childs ) {
                waitpid($_, 0);
         }
   }
[download]

The problem here is that it executes rsync for 5 folders at a time and wait until all of them finishes. BUT, I want to start rsync for 6th folder immediately after 1 of 5 rsync is completed.