There is no definition or some formula that makes perfume unisex. We are living in multicultural, cosmopolite world and each of us has its own taste and perception of things that can be influenced by trends, opinions or experiences. So basically each perfume, whether it is masculine or feminine, can be used by everyone.
Nevertheless, due to the powerful marketing there are some perfumes positioned as unisex that have been favored for a years beginning with Calvin Klein’s One since its creation 1994. Jean Paul Gaultier, Hermès, Acqua di Parma, Tom Ford, Dolce&Gabbana and many other Luxury brands own a masculine/feminine perfume these days.