Again, I'm only adressing the symptoms rather than thinking things through, but how's this?

#! perl -slw
use strict;
use 5.010;

our $FOO = 'fred';
sub FOO () { 'foo' }
BEGIN { undef $::{FOO}{CODE} }
say defined &FOO ? 'subroutine defined' : 'subroutine undef';
say eval 'FOO if $false; 1' ? 'FOO allowed' : 'FOO not allowed';
print $FOO;

__END__
C:\test>660264.p10
subroutine defined
FOO not allowed
fred
[download]