This is sort of a follow-up on a previous post asking for help with forks. I've been trying to learn, but I'm still very confused. I *think* I understand the basic idea, but the implementation still escapes me.

RichardK suggested I might be better off describing what I want to achieve so someone might make a suggestion as to the approach or module to use. So here it is in, pseudo-kind-of-code:

my @array1 = (1..5); # N=5
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
# - that I can do)
[download]

And here is my second attempt, using another module, but which still 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'm currently trying to read the documentation to other modules, but it's not easy-going... I really don't understand much, so I'd very happy if someone could put me on the right track.

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