package P2Pr;
sub new {
    shift();
    my $ref = bless {};
    $ref -> link( @_ ) if ( @_ ); # support one-stop calls
    return $ref;
}
sub link {
    my ( $obj, $relation, $other ) = @_;
    $obj -> { $relation } = $other;
}

# ... 
1;

# common or garden example ...

use P2Pr;
my $carriage1 = P2Pr -> new();
my $train = P2Pr -> new( 'afore', $carriage1 );
$carriage1 -> link ( 'abaft', $train );
my $carriage2 = new( 'abaft', $carriage1 );
$carriage1 -> link( 'afore', $carriage2 );