I only have a Camel II, which doesn't list arity in the operator table, so I'm not sure exactly what you are seeing.
However, arity has nothing to do with precedence; !$seen{$_}++ works because ++ has higher precedence than !.
Yes, i checked again (camel 3) and this time i understood the table corectly : precedence is shown by the way the operators are ordered, while "arity" is the number of argument they can take. thx .