Interesting. Will have to ponder that (later). Some factors, ranked in estimated descending order of effect:

• Because the thread does basically no work, the output thread is the bottleneck, and there's definitely less work being done there in testa.
• By using a hash instead of thread->join, we're moving work from the OS into Perl, which is going to be slower.
• Later tests are hurt by the variables created by the earlier tests. This should be tiny, though.
• You have next to no variables in your test program. That's going to help testa.

Here's another solution, but I doubt it'll catch up to testa.

my $request_q;
my $response_q;

sub testd {
    my $t0 = time;
    $request_q  = Thread::Queue->new();
    $response_q = Thread::OrderedQueue->new();

    my $merge_thread = threads->create(\&outputd);

    my @worker_threads;
    for (1..$threads) {
       push @worker_threads, async {
          while ( my $job = $request_q->dequeue() ) {
             $response_q->enqueue( $job->[0], worka($job->[1]) );
          }
       };
    }

    inputd();

    $request_q->end();
    $_->join() for @worker_threads;

    $response_q->end();
    $merge_thread->join();

    printf STDERR "testd done in %0.02f seconds\n",time - $t0;
}

sub inputd {
    my $id = -1;
    foreach my $a(1..$iterations) {
        my @chunk = ();
        foreach my $b(1..$chunksize) {
            my %adata = %data;
            $adata{'.'} = $a * $b;
            push(@chunk,\%adata);
        }

        $request_q->enqueue([ $response_q->get_token(), \@chunk ]);
    }
}

sub outputd {
    open(my $fh,'>',$aoutput);
    while( my $job = $response_q->dequeue() ) {
        foreach my $data(@$job) {
            foreach my $key(sort keys %$data) {
                print {$fh} $$data{$key};
            }
            print {$fh} "\n";
        }
    }

    close($fh);
}
[download]

Thread/OrderedDequeue.pm:

package Thread::OrderedQueue;

use strict;
use warnings;

use threads::shared;

sub new {
    my $class = shift;
    my $self = bless(\( my %self :shared ), $class);
    $self->{ q              } = \( my %q: shared );
    $self->{ last_given_ref } = \( my $last_given :shared = -1 );
    $self->{ next_read      } = 0;
    $self->{ ended          } = 0;
    return $self;
}

sub get_token {
    my $self = shift;
    lock(${ $self->{last_given_ref} });
    # We can't check if ended because we didn't lock %$self.
    return ++${ $self->{last_given_ref} };
}

sub end {
    my $self = shift;
    lock(%$self);
    $self->{ended} = 1;
    cond_signal(%$self);
}

sub enqueue {
    my $self = shift;
    lock(%$self);
    die("!!!") if $self->{ended};
    $self->{q}{ $_[0] } = shared_clone( $_[1] );
    cond_signal(%$self) if $_[0] == $self->{next_read};
}

sub dequeue {
    my $self = shift;
    lock(%$self);
    while (1) {
        if (exists( $self->{q}{ $self->{next_read} } )) {
            my $rv = delete( $self->{q}{ $self->{next_read}++ } );
            cond_signal(%$self);
            return $rv;
        }

        if ($self->{ended}) {
            cond_signal(%$self);
            return undef;
        }

        cond_wait(%$self);
    }
}

1;
[download]