Long story short is that a forward declaration doesn't create a glob. You can try this by doing

sub foo;
print ref \$::{foo}; # "SCALAR"

sub bar { 1 };
print ref \$::{bar}; # "GLOB"

sub baz;
print ref \$::{baz}; # "GLOB"
$baz = $baz;         # Just to mention the symbol "baz".
[download]

As it turns out if you inspect this a bit further the prototype of the subroutine (or "-1" if no prototype) is stored instead of a glob if no glob has been created already. When a symbol with the same name as the forward declaration a glob is created. So when you do *fn = $::{hello} you actually assign -1 to *fn making *fn aliased to *{-1}. (*{something} is just the "safe" way to write *something for symbols that aren't generally recognized as symbols by Perl and can be used with any sigil, like &{subname}.)

sub foo;
*{-1} = sub { 'oh' };     # Create a sub called "-1".
print &{-1}();            # "oh"
*bar = $::{foo};          # Really?
print defined &bar;       # "1" Huh? So now &bar is defined
                          # even though &foo isn't?
print bar();              # "oh"

#$foo;                    # But all this would change if you
                          # uncommented "$foo;".
[download]

Sneaky, eh?

ihb

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