Ensure that a platform has weaken support
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
Most notably this is RedHat Linux at time of writing, but other come and go
and do the same thing, hence "recurring problem".
The normal solution is to manually write tests in each distribution to
ensure that 'weaken' is available.
This restores the functionality testing to a dependency you do once in your
_Makefile.PL_, rather than something you have to write extra tests for each
time you write a module.
It should also help make the package auto-generators for the various
operating systems play more nicely, because it introduces a dependency that
they *have* to have a proper weaken in order to work.
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