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$a1 = [undef, item2];
$a2 = [item1, undef];

$a3 = [item1, undef];

$a3 = [item1, item2];

'LEFT_PRECEDENT' => {
       'SCALAR' => {
...
           'HASH'   => sub { _merge_hashes( $_[0], $_[1] ) },
       },
   },

sub _merge_arrays {
   my ( $left, $right ) = ( shift, shift );
...
       }
   }#for     return \@newarray;
}