On my Debian Etch I get the incorrect result a1 from perl shipped with Debian.

$ perl -v
This is perl, v5.8.8 built for i486-linux-gnu-thread-multi
[download]

But it seems to be fixed in 5.9.5 (correctly produces ab):

This is perl, v5.9.5 DEVEL31566 built for i686-linux