Iirc, just need to enter. I’ll check it later.
Checked it now. Yes, just re-entering once in 5 years seems to be enough.
Article 18
Where a foreign professional, after having obtained approval for permanent residence from the National Immigration Agency, Ministry of the Interior, leaves the State for more than five years without re-entering, the National Immigration Agency, Ministry of the Interior, may revoke said person’s permanent residence permit and cancel said person’s Alien Permanent Resident Certificate, and the provisions of Article 33 Paragraph 1 Subparagraph 4 of the Immigration Act do not apply.