Library modules should not in general export built-in names like open or chdir as part of their default @EXPORT list, because these may sneak into someone else’s namespace and change the semantics unexpectedly. Instead, if the module adds that name to @EXPORT_OK, then it’s possible for a user to import the name explicitly, but not implicitly. That is, they could say
use Module 'open';
and it would import the open override. But if they said
they would get the default imports without overrides.
The foregoing mechanism for overriding built-in is restricted, quite deliberately, to the package that requests the import. There is a second method that is sometimes applicable when you wish to override a built-in everywhere, without regard to namespace boundaries. This is achieved by importing a sub into the special namespace CORE::GLOBAL::.
Makeshifts last the longest.