Perry should order a flotilla of boats to patrol the Rio.
He has already done that in some areas. They mount automatic weapons, but they appear to be .223/5.56. It wouldn't take much for them to be outgunned if the cartels so desired. After all, the cartels have a lot of military grade weapons and explosives available. An ambush would probably be easy, using .50 cal., RPGs, mortars, etc. Then it would be open war on the border - not that we already aren't almost there with massive armed groups of cartel soldiers there already operating freely.
The only thing keeping the cartels from shooting is the feds are letting them have their way everywhere and are almost completely ineffective in stopping anything so the drug and coyote money just keeps rolling in. If anybody tries to stop that in a serious way things will likely turn hot.