In Mail::Audit you should not remove the line, instead if you're on Perl 5.6 or better, it should use WeakRef's weaken (shipped in the core of Perl from 5.6 on) to fix the reference counting.

As for Devel::Leak, it may well use an internal data structure for this. If you're curious, peaking at sourcecode is helpful. In any case the problem is that Devel::Leak::NoteSV should clean up the results of having being called before if it is called again. If you can't puzzle out from the code how you might do this, the bug description you started with is specific enough for the author to probably figure it out.

But just in case, it never hurts to attach a complete description of your environment to a bug report just in case it matters. perl -V will give you such a description.