package Foo;
no circular::loading;

# ...

1;

This is a rather simple module. You use it at the top of your file right after your package declaration (this is important). It will throw an error including a stacktrace if the package is required again without having been fully compiled. It does that by deferring the entry in %INC that usually happens at load time until the end of compilation time with the help of B::Hooks::EndOfScope.

The package's %INC entry will not be available until after compile time. This means the package won't be seen as loaded by the packages used after no circular::loading.

Another non-nicety is that you'll have to put the no circular::loading declaration first in your package as to not import from other packages multiple times. This might not break anything, but it might hide the real circular error from you.

This is the method called when

no circular:loading;

is used. There is no corresponding import method at the moment.