Generally speaking, returning $self is by no means necessary. It's a convenience, as it allows

my $d = Date->new();
$d->year(2010);
$d->month(3);
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to be written as

my $d = Date->new()
   ->year(2010)
   ->month(3);
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In your case, _calculate_foo is only called internally and its result is discarded. There's no reason to return $self.

If you continued to have _calculate_foo return $self, you could simplify calculate_foo. But since you're likely going to use the value you calculated, why not return it instead?