One recurring problem in modules that use Scalar::Util's weaken function
is that it is not present in the pure-perl variant. While this isn't
necessarily always a problem in a straight CPAN-based Perl environment,
some operating system distributions only include the pure-Perl versions,
don't include the XS version, and so weaken is then "missing" from the
platform, despite passing a dependency on Scalar::Util successfully.