KMnO4(aq) + HCl(aq) -----> KCl(aq) + MnCl2(aq) + H2O(l) + Cl2(g)?

This is an oxidation-reduction reaction, so the Cl2 comes from oxidation of the HCl by the KMnO4. The manganese is reduced from the +7 oxidation state to the +2 state. No, it is not a property of a strong acid --- it's simply the ability of permanganate (MnO4-) to oxidize the Cl- ion.