duck.pl

#!/usr/bin/env perl

use Duck;

my $duck = Duck->new({ name => 'hellangel' });

print $duck->quack;
print $duck->quack('Wow, this rocks!');

$duck->age(3);
print 'my duck\'s age: ' . $duck->duck_age . ' (' . $duck->age . ")\n";

Duck.pm

package Duck;

use strict;
use warnings;
use base qw( Class::Accessor );

__PACKAGE__->mk_accessors(qw( name age ));

sub duck_age {
    my ( $self ) = @_;
    return $self->age * 17;
}

sub quack {
    my ( $self, $message ) = @_;

    return "Quack: $message\n" if defined $message;

    return 'Hi, this is the duck ' . $self->name . "\n";
}

!! 42; # must. return. true value
__END__