I see no reason to mess with globs :)

C:\>more 2
sub FORWARD_DECLARATION;

BEGIN {
   print 'glob ', defined *::FORWARD_DECLARATION{CODE};
   print 'defined & ', defined &FORWARD_DECLARATION;
   print 'exists & ', exists &FORWARD_DECLARATION;
}

sub FORWARD_DECLARATION { 5 }
print 'glob ', defined *::FORWARD_DECLARATION{CODE};
print 'defined & ', defined &FORWARD_DECLARATION;

C:\>perl -l 2
glob 1
defined &
exists & 1
glob 1
defined & 1

C:\>
[download]