For what it's worth, your well written test program allowed me to determine the code that changed the tainting behaviour: it was patch 24010:

Change 24010 by rgs@bloom on 2005/03/08 17:53:50

Subject: Encoding neutral unpack
From: perl5-porters@ton.iguana.be (Ton Hospel)
Date: Sun, 6 Mar 2005 18:29:38 +0000 (UTC)
Message-Id: <d0fi6i$k06$1@post.home.lunix>

It's quite a large patch: Show patch