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.
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.
To get around that for this module, it will do a few dirty tricks.
If Scalar::Util is not available at all, it will issue a normal dependency on the module. However, if Scalar::Util is relatively new ( it is >= 1.19 ) and the module does not have weaken, the install will bail out altogether with a long error encouraging the user to seek support from their vendor (this problem happens most often in vendor-packaged Perl versions).
This distribution also contains tests to ensure that weaken is available using more normal methods.
So if your module uses "weaken", you can just add the following to your Module::Install-based Makefile.PL (or equivalent).
requires 'Task::Weaken' => 0;
This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
The full text of the license can be found in the LICENSE file included with this module.