#!/usr/local/bin/perl
use strict;
use warnings;

use Player;
my $player = Player->new('vx' => -1);
for (keys %$player) {
        print "$_: ", $player->{$_}, $/;
}

##</code><code>##

package Player;
use Mover;
@Player::ISA = qw(Mover);
use strict;

sub defaults {
        my $defaults = {
                'symbol'   => '_',
                'score'    => 0,
        }
}

1;

##</code><code>##

package Mover;
use Object;
@Mover::ISA = qw(Object);
use strict;

sub defaults {
        my $defaults = {
                'symbol' => '?',     # override this in subclasses
                'x'      => 0,       # x-coordinate
                'y'      => 0,       # y-coordinate
                'vx'     => 0,       # velocity in x-direction
                'vy'     => 0,       # velocity in y-direction
                'shown'  => 1,       # still alive?
        }
}

1;

##</code><code>##

package Object;
use strict;

sub new {
        my ($caller, %args) = @_;
        my ($self, $class);
        $class = ref $caller;
        $class ||= $caller;
        $self = bless {}, $class;
        $self->init(\%args);
        return $self;
}

sub init {
        my ($self, $args) = @_;
        my ($defaults);

        # See if superclass has defaults
        if ($self->can('SUPER::defaults')) {
                $defaults = $self->SUPER::defaults();
        }
        # Let current class override superclass
        if ($self->can('defaults')) {
                my $current_defaults = $self->defaults();
                for my $attr (keys %$current_defaults) {
                        $defaults->{$attr}
                             = $current_defaults->{$attr};
                }
        }
        # Set default values
        if (ref($defaults) eq 'HASH') {
                for my $attr (keys %$defaults) {
                        $self->{$attr} = $defaults->{$attr};
                }
        }
        # If %args are given to new(), use those instead
        for my $attr (keys %$args) {
                $self->{$attr} = $args->{$attr};
        }
}

1;

##</code><code>##

$ ./test.pl
score: 0
vx: -1
symbol: _