A perfect hack is when you press
in the sweet spot of a hacking attempt (which is generally the yellow, whereas the orange around it is just a general access spot). In order to get this, you must hack at least fifteen items by getting all perfects during that attempt. As you only need to get fifteen, the chances are rather high that you might get this early on during your run through Seoul; with the Legion Lairs, operations, and everything else that brings you to a hacking prompt, just be careful and take your time.
And remember: getting Perfects gives you experience, so why not?!