Usually when you are writing a method you make the return value $self, particularly important if you have modified one or more of the objects data members.

Usually? The majority of the code I see doesn't. And important? It allows you to write:

$obj->name("John Doe")->birthday("1969-07-20")->income(53_389.86);
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but I see most people write that as

$obj->name("John Doe");
$obj->birthday("1969-07-20");
$obj->income(53_389.86);
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even if they could chain. I wouldn't call it "important". And you really have to play dirty tricks if all your methods would return $self. Accessors to get the value of attributes are a pain in the ass to write (and use) if all they are supposed to return is $self.