I browsed thru various perldelta513* w/o spotting it.

For those interested here a one-liner for testing your perl-version:

lanx@nc10-ubuntu:~$ perl -e '@k=( %h=(a=>42,b=>2,a=>1) ); print "$] Bu
+g: @k\n" unless @k ~~ [%h]'
5.010000 Bug: b 2 b 2
[download]

I'll gonna update a non-smartmatch solution here...

UPDATE:

w/o smart-match for even older perl versions (maybe the bug was introduced somewhere)

lanx@nc10-ubuntu:~$ perl -e '%h2=@k=( %h=(a=>42,b=>2,a=>1) ); delete @
+h{keys %h2}; print "$] Bug: @k\n" if %h'
5.010000 Bug: b 2 b 2
[download]

UPDATE: replace version-vars s'$^V'$]'g in code-sections