#! 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