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)

##</code><code>##

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
}