fork (and in turn, Parallel::ForkManager) doesn't return data to the parent process and has no provisions to do so.

The easiest approach would be to have each client write out the results to a file, and then read each file in turn in the parent. The run_on_finish callback allows you to conveniently know when to collect more data in the parent:

use JSON::XS 'encode_json', 'decode_json';

$pm->run_on_finish(sub {
      my ($pid, $exit_code, $filename) = @_;
      print "** $ident was just written";
      open my $fh, '<', $filename or die "Couldn't read child results 
+from '$filename': $!";
      binmode $fh;
      local $/;
      my $results = <$fh>;
      # Convert results back to the data structure:
      $results = json_decode( $results );

      # now, merge those results with the other results you already ha
+ve
});

...

for my $task (@tasks) {
    my $filename = "some filename based on $task";
    $pm->start( $filename ) and next;
    ... do the work
    open my $fh, '>', $filename
        or die "Couldn't write output file '$filename': $!";
    binmode $fh;
    print $fh json_encode(\%results);
};
[download]