Thanks for all of the answers.. I do not want to use Parallel::ForkManager because I am not quite sure how to ship it with my script (this is a homework, but do not worry, I am doing an "extra" feature in the homework so it is okay (I can run 20 children processes at the same time, no one cares, but i want to do ~4 by ~4 when possible).

I am just not sure how to fork ~4 different processes. Below is the code I just wrote.. 
while(scalar(@directories) > 0) {
    
    #Take first 4 when there are 4 or more elements in the array
    if(scalar(@directories)>=4){
        @part= @directories[0..3];
    }    
    
    #Take as many as possible when <4 elements in the array
    elsif(scalar(@directories)<4){
        @part = @directories [0..scalar(@directories)];

    }
    
    #Shorten the array by the number of elements taken in @part
    for(my $i=0; $i<scalar(@part); $i++) {
        shift(@directories);
    }
    
    #For each element in @part, create children processes
    #************************************
    foreach my $f (@part) {
        
        my $secdirtoget= "${passed_dir}/StretchingDecaAlanine/GMXCubic
+Box/Umbrella/${f}";
        chdir ($secdirtoget);
    
        system ("ln -s ../md_umbrella.mdp ../index.ndx ../topol.top ..
+/posre_CTerm.itp ../posre.itp .");
        system ("grompp -f md_umbrella.mdp -c pullconf.gro -n index.nd
+x");
        exec 'mdrun', '>logje 2>&1 &';
        
        chdir ("${passed_dir}/StretchingDecaAlanine/GMXCubicBox/Umbrel
+la");    
    }
    #************************************
}

[download]
The thing is that I want my script to wait until 4 or less mdrun calls in the foreach my $f (@part) finish doing their job, and then continue with the next 4 or less elements when a new partition in the beginning of the while is made.. Many thanks..

In reply to Re^2: Splitting an array into subarrays of size ~n by hotel
in thread Splitting an array into subarrays of size ~n by hotel

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