Thanks, I didn't get the crucial meaning about exiting in the documentation. Learning to understand the documentation is also learning - I remember how I struggled with man pages years ago :-/

Here's what I want to do in pseudo-kind-of-code, apart from a bunch of extra loops that shouldn't have any influence.

my @array1 = (1..5); # N
my @array2 = qw/a b/;

for my $val1 (@array1) {
    # Start N processes here that can run in parallel
    # Each process outputs data to its own separate file
    # I will need it in the future

    for my $val2 (@array2) {
        # For each of the N processes, wait until it is done, 
        # then start 2 parallel processes which use
        # the output data as input
        # Save the output of each process separately to 2N files
        # (two files with N elements would be better, but as I 
        # couldn't figure that out, I just postprocess the data ;-)
    }
}
# "waitallchildren" or equivalent

# Postprocess step to reduce the final output to 2 files
[download]

I just tried this, but it doesn't work. The outer part does, but as soon as I uncomment the inner part, my prompt doesn't return anymore. No idea what's going on.

use Proc::Fork;
for my $val1 (@array1) {
    run_fork {
        child {
            open FILE, ">$val1.txt";
            print FILE "Output of step 1\n";
            close FILE;
        }
        parent {
            my $child_pid_outer = shift;
            waitpid $child_pid_outer, 0;

            # for my $val2 (@array2) {
            #     run_fork {
            #         child {
            #             open FILE1, "$val1.txt";
            #             open FILE2, ">$val2$val1.txt";
            #             while (my $line = <FILE1>) {
            #                 $line =~ s/1/2/;
            #                 print FILE2 $line . "\n";
            #             }
            #             close FILE1;
            #             close FILE2;
            #         }
            #         parent {
            #             my $child_pid_inner = shift;
            #             waitpid $child_pid_inner, 0;
            #         } # Parent inner loop
            #     }; # Inner fork
            # } # For loop

        } # Parent outer fork
    }; # Outer fork
}
[download]

I did have a (short) look at other packages, but I must admit I didn't understand much, unfortunately.

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