Remove the brackets from the my statement in the while condition.

   while( my $returned_value = $queue->dequeue   ) {
   #        ^               ^
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The code will then behave as you expect.

Update: Sorry! Wrong answer. I hadn't reinstalled Thread::Queue::Any since ungrading and tried it with Thread::Queue instead.

P:\test>type 319494.pl8
use strict;
use threads;
use Thread::Queue;

my $queue = Thread::Queue->new;

my $prod_thread = new threads(\&producer, $queue);
my $cons_thread = new threads(\&consumer, $queue);

$prod_thread->join;
$cons_thread->join;

sub producer {
        my $queue = $_[0];
        my $value = 1234;
        $queue->enqueue( $value );
        $queue->enqueue( undef );
}

sub consumer {
        my $queue = $_[0];
        while( my $returned_value = $queue->dequeue   ) {
                printf "I have been returned: %i\n", $returned_value;
        }
}

P:\test>319494
I have been returned: 1234
P:\test>
[download]

It worked! ...but not with Thread::Queued::Any :(