From @Skarsnik

As the title say. It could probably useful to have a syntax/keyword to specify when the module depends on something that is not another perl6 module. Including the version/other

There is multiple use for that​:
Installer can warn the user/admin that they will need something else in order for the module to work.
Tools like dh-make-perl (a debian tools that build .dpkg from cpan/source) could add this as dependency if needed.

Could be like​:
extern-depends { "C​:libfoo​:ver<3.*>", "Python​:bar​:ver<2.1.2>" }