Your mistake is here  use overload '+' => \&add_latency,

add_latency is not written to cooperate with overload

As overload examples  my ($self, $other, $swap) = @_;

To explain, + is a binop, binary operator, it takes two arguments ... this arglist is slightly clearer

my( $self_operand_overloaded, $other_operand, $other_operand_was_on_the_left_side ) = @_;

$other_operand can be undef

$other_operand_was_on_the_left_side is a boolean, not an object

overload explains in detail