The simple fact is that not only do you not have to declare global variables -- "fully qualified variable names" are always globals --, you cannot declare them. Perl has no mechanism for doing that.
You seem to be equating the action of our with that of my, but they are in not equivalent. my actually creates new variables, but our simply allows you silent, unqualified access to them in the current lexical scope.
One possibility that might in some way meet your requirement would be to use Hash::Util::lock_keys() to restrict the package hash to just those keys you require.
Update: A little explanation of the idea:
use Hash::Util qw[ lock_keys ];; $My::Space::fred = 12345;; %My::Space::hash = 1 .. 10;; @My::Space::array = 1 .. 10;; lock_keys( %My::Space:: );; print $My::Space::fred;; 12345 print $My::Space::fred = 23456;; 23456 print $My::Space::fred;; 23456 print $My::Space::freddy = 1234;; Attempt to access disallowed key 'freddy' in a restricted hash at (eva +l 18) line 1, <STDIN> line 10.