Your system may not be setup to use weaken properly.

The ability to weaken references is built into Perl itself, and none of those modules would affect that.

I believe you are referring to the distro that shipped with a broken Scalar::Util package. It didn't cause weaken to work improperly, it caused the function weaken to throw an exception when used. This is clearly not the OP's problem. (That problem was fixed by simply reinstalling Scalar::Util from source.)