I'm not familiar enough with the general syntax specs for connection strings, let alone the specific syntax variants supported by Oracle and/or DBD::Oracle, to say why the first $DSN value works and the second does not. (With reference to stevieb' reply above, I assume that:

my $DSN='dbi:Oracle:host=127.0.0.1:sid=db.company.local:port=1521';
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yields a failure even when you try it first.) In any case, given that you have a form of $DSN that works, is there a compelling reason for finding a different form that also works?

(I wouldn't be inclined to regard the failure of the above syntax to be "unusual" or "surprising" - probably because I don't have enough relevant experience to expect it to work. But having found a method that works, I'd just use that and stop worrying about it.)