That doesn’t surprise me at all. It’s a feature, not a bug: the same behaviour makes sub foo; predeclarations work, as well as things like

my $x = \&foo;

*foo = sub { 'yep' };

print $x->(), "\n";

__END__
yep
[download]

The only solution for what you want that I can think of is to save all the slots from the glob, then blow it away, and finally restore the other slots.

Makeshifts last the longest.