package Tank{
    use strict;
    use warnings;

    # Tank bauen
    sub new{
        my $class = shift;
        my %cfg = ( fuel => 0, @_);
        return bless \%cfg, $class;
    }

    # Tankinhal ändern
    sub fuel{
        my $self = shift;
        my $fuel = shift;
        unless( defined $fuel ){ return $self->{fuel} }    
        else{ $self->{fuel} = $fuel }
    }
};

package PKW{
    use strict;
    use warnings;

    # Auto bauen
    sub new{
        my $class = shift;
        my %cfg  = (tank => {}, @_);
        bless \%cfg, $class;
    }

    # Tankinhalt ändern, delegierte Methode
    sub fuel{
        my $self = shift;
        return $self->{tank}->fuel(@_);
    }
};

1; ##########################################################
package main;

use strict;
use warnings;

my $tank = Tank->new( fuel => 100 );
my $pkw = PKW->new( tank => $tank );
my $zweitwagen = PKW->new( tank => $tank );
$zweitwagen->fuel(0); # Leerfahren

print $pkw->fuel; # Tank leer