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

##</code><code>##

$a3 = [item1, undef];

##</code><code>##

$a3 = [item1, item2];

##</code><code>##

'LEFT_PRECEDENT' => {
       'SCALAR' => {
           'SCALAR' => sub { $_[0] },
           'ARRAY'  => sub { $_[0] },
           'HASH'   => sub { $_[0] },
       },
       'ARRAY' => {
           'SCALAR' => sub { [ @{$_[0]}, $_[1] ] },
           'ARRAY'  => sub { [ @{$_[0]}, @{$_[1]} ] },
           'HASH'   => sub { [ @{$_[0]}, values %{$_[1]} ] },
       },
       'HASH' => {
           'SCALAR' => sub { $_[0] },
           'ARRAY'  => sub { $_[0] },
           'HASH'   => sub { _merge_hashes( $_[0], $_[1] ) },
       },
   },

##</code><code>##

sub _merge_arrays {
   my ( $left, $right ) = ( shift, shift );
   if( ref $left ne 'ARRAY' || ref $right ne 'ARRAY' ) {
       carp 'Arguments for _merge_arrays must be array references';
       return;
   }

   my $length = $#$left;
   $length = $#$right if ($#$right > $length);
     my @newarray;
   for (my $num = 0; $num <= $length; $num++) {
       if ( defined $left->[$num] && defined $right->[$num] ) {
           @newarray[ $num ]
               = merge ( $left->[ $num ], $right->[ $num] );
       }
       elsif ( defined $left->[$num] && !(defined $right->[$num]) ) {
           @newarray[ $num ]
               = $left->[ $num ];
       }
       elsif ( !(defined $left->[$num]) && defined $right->[$num] ) {
           @newarray[ $num ]
               = $right->[ $num ];
       }
       else {
           $newarray[$num] = $clone_behavior
               ? _my_clone( $left->[$num] ) : $left->[$num];
       }
   }#for     return \@newarray;
}