use strict;
use warnings;

open(my $fh, '<', $0);  # Calls CORE::open

use override open => sub { print "Hi\n"; };
open(my $fh2, '<', $0); # Prints Hi

use override open => sub { print "Ho\n"; };
open(my $fh4, '<', $0);  # Prints Ho

use override open => undef;
open(my $fh5, '<', $0);  # Calls CORE::open