I've done no real testing (my food is ready), but is this the effect you are looking for? 

#! perl -slw
use strict;
use 5.010;

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

__END__
C:\test>660264.p10
subroutine undef
FOO not allowed

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In reply to Re: Removing CODE slot in typeglob / Reversing "use subs ...;" by BrowserUk
in thread Removing CODE slot in typeglob / Reversing "use subs ...;" by lodin

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