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

       sub my_sub {
        $hohref = shift;       
...
        # wait for all threads to end
        $pool->join();
        }

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

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