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)