It has something to do with BEGIN

In case the BEGIN is there, it is guarantied to execute that block first, so before it sees $x (or in this case actually $main::x) and by that time it exists because the block has already been executed.

Yes, I concur - and an interesting point regarding the warnings. I assume they don't occur because otherwise they'd get triggered by the import mechanism regularly, but it'd be interesting to find any docs on that, too...