1+$duration is not an explicit type conversion, it is explicit addition that implies a type conversion, if required.
+ is a monomorphic operator. So is .. Perl values are polymorphic.
Arguing that operator-enforced coercions in Perl are implicit is silly nonsense; to do so you must assume a priori that values should be monomorphic.
I can write 1+$duration with no intention of doing a conversion.
You can also write $string + $duration with no intention of performing addition, but that has nothing to do with typing and everything to do with you writing buggy (or poorly understood) code.