So you are saying that there is nothing stopping linking of the entire GNU compiler source code via Perl, and then the releasing of a closed source code generator?
That's not what he's saying at all. Your example is quite obviously
a derived work of GPL'd software.
What he's saying is that most perl modules are distributed under
the dual license scheme, and so this isn't an issue for the majority
of cases. However, we do need to be aware that there are some
perl modules on CPAN that are just GPL'd, and are not dual licensed,
so you need to be aware of those.