PERSEC'd info is often personal information about a person/soldier/civilian that, if leaked or passed on to the wrong party, could result in either harm, defamation, or potentially even death to said person. As such, the principles of both OPSEC and PERSEC are taught to pretty much every recruit early on in-training as basic counter-intelligence.
Random stranger: "Where are you stationed?"
Random stranger: What's your SSN/mother's maiden name/proficiency with certain weapons?"
Soldier: "Sorry mate. That's PERSEC'd info. Now fuck off."