How about the property that the width and height (of a Rectangle) are independent: 
package Rectangle;
use Moo;

has height => (is => 'rw');
has width  => (is => 'rw');

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

    printf "Demo of %s\n", ref $self;
    $self->show_dimensions;
    printf "Resetting height => 4, width => 3\n";
    $self->height(4);
    $self->width(3);

    $self->show_dimensions;
    printf "area: %d\n", $self->area;
    $self->show_dimensions;
}

sub area {
    my ($self) = @_;
    $self->{width} * $self->{height};
}

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

    printf "height: %s, width: %s\n", $self->{height} || 'undef', $sel
+f->{width} || '';
}

package Square;
use Moo;

extends 'Rectangle';

sub area {
    my ($self) = @_;
    $self->{height} = $self->{width};
    $self->SUPER::area();
}

package main;

my $rect = Rectangle->new(width => 2, height => 2);
my $sq = Square->new(width => 2);

$rect->demo;
print "\n";
$sq->demo;

[download]
Which outputs: 
Demo of Rectangle
height: 2, width: 2
Resetting height => 4, width => 3
height: 4, width: 3
area: 12
height: 4, width: 3

Demo of Square
height: undef, width: 2
Resetting height => 4, width => 3
height: 4, width: 3
area: 9
height: 3, width: 3

[download]
The square fails as a stand-in for a rectangle. The is one of the reasons that wise man say Favor Composition Over Inheritance

In reply to Re^4: When to Use Object Oriented approach in Perl? (RFC) by Arunbear
in thread When to Use Object Oriented approach in Perl? (RFC) by thanos1983

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