Yes, I'm aware of that. The point of this thread is that IO::AIO had no problems before the upgrade to XCode 4. At first glance it appears IO::AIO is sloppy in that it uses symbols without exporting them from Fcntl and POSIX, but I think it was working before because the XS code magically exported the symbols so they were available on the Perl side. After the upgrade, the mechanism that exported those symbols from XS to Perl appears to not work.