sub helper {
        my $href = shift;
        $href->{NEW_KEY}=1;
       }

##</code><code>##

       sub my_sub {
        $hohref = shift;       
    
        # create thread pool
    	my $pool = Thread::Pool::Simple->new(
    		do  => [ \&helper ]
    	);
    
    	# submit jobs
    	foreach my $hashref ( values %{$hohref} ) {
    		$pool->add( $hashref );
    	}
    
    	# wait for all threads to end
    	$pool->join();
        }

##</code><code>##

	foreach my $hashref ( values %{$hohref} ) {
		helper( $hashref );
	}

##</code><code>##

use strict;
use warnings;

use threads;
use threads::shared;

use Thread::Pool::Simple;
use 5.010;

use Data::Dumper;

sub helper {
	say "helper starts";
	my $href  = shift;
	say "href is $href";
	$href->{NEW_KEY} = 1;
	say "helper ends with $href";
}


sub my_sub {
	my $hohref = shift;
	
	my $shared_hohref = shared_clone $hohref;
	my $pool = Thread::Pool::Simple->new( do => [\&helper] );

	# submit jobs
	foreach my $hashref ( values %{$shared_hohref} ) {
		say "adding to pool: $hashref";
		$pool->add($hashref);
	}

	# wait for all threads to end
	$pool->join();
	
	return $shared_hohref;
}

my $hoh = {
	A => { NAME => "a" },
	B => { NAME => "bb" }
};

say "1\t", Dumper $hoh;
my $updated_hoh = my_sub($hoh);
say "2\t", Dumper $updated_hoh;