I also noticed that the indirect method syntax for push doesn't work inside the BEGIN block. I wasn't sure if that was a feature or something still to be done.
It's a well-known bug. Unfortunately we have some roadmap items to complete before we can fix that - we need to have static lexical pads at compile time to make built-in functions available in BEGIN blocks.
Perl 6 - links to (nearly) everything that is Perl 6.