sub positional_mutator(&;$) {
    my ($worker,$name) = @_;
    no strict 'refs';
    *{$name} = sub {
        my $self = shift;
        my $obj;
        if (defined wantarray) {
            $obj = $self->copy();
        } else {
            $obj = $self;
        };
        $worker->($obj,@_);
    }
};

positional_mutator {
  my ($obj,@args) = @_;
  ... calculate new position here
}, 'frobnicate_position';