Yes, you should. What's in core varies from one version of perl to another. Normally it's just things being added so modern perls have more in core than older ones, but occasionally things get removed. CGI.pm, for example, may be removed soon.

What's more, you should list every module your code relies on. If your code uses Foo::Bar and Baz::Barf, but Foo::Bar depends on Baz::Barf, you should declare a dependency on both of them, because at some point in the future Foo::Bar might stop depending on Baz::Barf and use something else instead.

In general, if your code mentions a module, you should list it as a pre-requisite.