Yes, I've stumbled upon this before and found it odd. The closest thing I can find to documentation for it is in perlmod:

"The results of creating new symbol table entries directly or modifying any entries that are not already typeglobs are undefined and subject to change between releases of perl."

... which may or may not cover this situation.

It seems to be that if the symbol table is being manipulated by code in another package, Perl thinks you're importing a variable, and treats that as "as good as a declaration", but if it's manipulated by code within the same package, it doesn't.

Sorry if that's a bit of a non-answer.

Update: take a peek at this:

use Devel::Peek;

BEGIN {
  *main::foo1 = [];
  package X;
  *main::foo2 = [];
};

Dump($_) for \*main::foo1, \*main::foo2;
[download]

In particular, look at the FLAGS.