G'day morgon,

I believe you're making incorrect assumptions about CORE::GLOBAL. Here's a repeat of your examples along with equivalents not involving CORE::GLOBAL.

$ perl -wle 'print \&CORE::GLOBAL::sleep'
CODE(0x224fcb8)
$ perl -wle 'print \&CORE::GLOBAL::hubbabubba'
CODE(0x1d4acb8)
$ perl -wle 'print \&hubbabubba'
CODE(0x255bcb8)
$ perl -wle '&CORE::GLOBAL::sleep(1)'
Undefined subroutine &CORE::GLOBAL::sleep called at -e line 1.
$ perl -wle '&CORE::GLOBAL::hubbabubba(1)'
Undefined subroutine &CORE::GLOBAL::hubbabubba called at -e line 1.
$ perl -wle '&hubbabubba(1)'
Undefined subroutine &main::hubbabubba called at -e line 1.
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As you can see, the outcomes are the same with or without CORE::GLOBAL.

I can't really comment on internals: I'm not sufficiently versed in that area. There are, however, plenty of monks who can help with that.