Perfect technique, in my opinion, is the ability to sing from a pure falsetto to a full-voiced, ringing, balanced sound on any pitch and on any vowel without feeling or hearing where the shift between the two happens. This is called messa di voce, going from the softest pianissimo to the loudest forte and back again without any trace of a register shift.
Messa di voce is almost impossible without complete freedom and flexibility in the voice, and I’ve heard no singer in the world who does it perfectly on every pitch and vowel throughout the range.
Smith, W. Stephen, and Michael Chipman. The naked voice: A wholistic approach to singing. Oxford University Press, 2007.