The channel object is bidirectional. Therefore, the worker can send its PID value to the producer. This has the effect of max one item in the channel. The producer writes to STDOUT like the demonstration by karlgoethebier.

#!/usr/bin/env perl

use strict;
use warnings;
use MCE::Hobo;
use MCE::Channel;
use feature qw(say);
use constant AMOUNT => 0.001;

my $cores = MCE::Util::get_ncpu();
my $queue = MCE::Channel->new();

MCE::Hobo->init(
    void_context => 1,
    posix_exit => 1,
);

# Consumers

sub consumer_task {
    while ( my $input = $queue->recv() ) {
        my $id = $input->{id};
        $queue->send2( MCE::Hobo->pid() );
        qx( sleep @{[ AMOUNT ]};) 
    }
}

MCE::Hobo->create( \&consumer_task ) for 1..$cores;

# Producer

for my $id ( 1..800 ) {
    $queue->send({ id => $id });
    my $hobo_pid = $queue->recv2();
    say qq($id ) . $hobo_pid;
}

$queue->end();

MCE::Hobo->wait_all();
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