in reply to Re: What does this mean?
in thread What does this mean?

defined $id and $$self{_id} = $id; or with newer perls $$self{_id} //= $id;

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Re^3: What does this mean?
by moritz (Cardinal) on Aug 21, 2012 at 07:14 UTC
    $$self{_id} //= $id; checks the definedness of $$self{_id}, not of $id (as the other suggested snippets do), so it does something completely different (never reset the attribute, instead of prevent assignment of an undefined id).