I did a little searching, and did find a proposal from 2008 about anonymous packages for 5.11/5.12. It doesn't look like anything came from that proposal, though. Whoever originally posted about anonymous packages being a new feature seems to have been misinformed.

In reply to Re: info on anonymous packages in perl-5.12.0 needed by webfiend
in thread info on anonymous packages in perl-5.12.0 needed by sergstesh

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