Inheritance can be discouraged by using the one arg form of bless or by using the technique of using separate packages for class and instance methods described in Re: OO - best way to have protected methods (packages). Here is another example:

use strict;
package Queue;

sub new {
    bless [], 'Queue::__Internal';
}

package Queue::__Internal;

sub push {
    my ( $self, $item ) = @_;

    push @{$self}, $item;
}

sub pop {
    my ( $self ) = @_;

    shift @{ $self };
}

sub size {
    my ( $self ) = @_;

    scalar @{ $self };
}

1;
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