Description:

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.

Most notably this is RedHat Linux at time of writing, but other come and go
and do the same thing, hence "recurring problem".