{   package A;

    my $self = {
            key_1 => value_1,
            key_2 => value_2,
            key_3 => [objects of B],
            key_n => 'value_n',
        };

    sub lastIndex {
        my $self = shift;
        return $#{$self->{key_3}};
    }
}

{   package B;
    
    my $self = {
            key_1 => value_1,
            key_2 => value_2,
            key_n => 'value_n',
        };
    
    sub isLast {
        my $self = shift;
        
    }
}

my $pkg_B_[4]->isLast;