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 necesarily 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.