before q{limit} => sub {
        my $self = shift;
        if ( scalar @_ ) { 
            my $val = shift;
            if ( ref $val eq q{HASH} ) {
                foreach my $k ( keys %{$val} ) {
                    if ( defined $val->{$k}->{$l} ) {
                        $self->limit->{$k}{$l} = $val->{$k}->{$l};
                    }
                    else {
                        my $result = $self->limit->{$k}{$l};
                        return $result;
                    }
                }
            }
        }
    };
    __PACKAGE__->meta->make_immutable;
    1;

    package main;

    my $wall = Boundary->instance();
    $wall->limit( { min => { x => 0, y => 0, }, 
                    max => { x => 639, y => 478, }, }, );
    # ...
    say $wall->limit( { min => { x => undef, }, }, );
    # Expected: 0
    # Actual: HASH(0x801a440b8)
    say $wall->limit( { max => { x => undef, }, }, );
    # Expected: 639
    # Actual: HASH(0x801a44160)