# The package name will become the class name
package Square; 

use strict;
use warnings; # Always a good idea

# public Square new(double size)
# Instantiate a square of a specified size.
sub new
{
   my($package, $size) = @_;

   die unless scalar @_ == 2;

   my $this = [
      $size # Length of side of the square
   ];

   bless $this, $package or die;

   return $this;
}

# private double _GetSize()
# Get the side length. This method is private, denoted by the underscore.
# This is not enforced, but at least it is clearly indicated.
sub _GetSize
{
   my($this) = @_;

   die unless scalar @_ == 1;

   return $this->[0];
}

# public double Area()
# Get the area of the square.
sub Area
{
   my($this) = @_;

   die unless scalar @_ == 1;

   return $this->_GetSize() * $this->_GetSize();
}

1; # Package needs to return true