use Proc::Queue size => 8;

for my $dir (@dirs) {
  run_back {
    chdir $dir and exec "foo_matrix";
  };
}

1 while wait > 0;
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Proc::Queue is a single file module that can be installed just dropping it in some place under your home (for instance, as /home/robert/lib/perl/Proc/Queue.pm, and then on your script use lib '/home/robert/lib/perl';)