use strict;

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

BEGIN {
        *{main::open} = sub { print "Hi\n"; };
        use subs 'open';
}

open(my $fh2, '<', $0); # Prints Hi

BEGIN {
        delete $main::{open};
}

BEGIN {
        *{main::open} = sub { print "Ho\n"; };
        use subs 'open';
}

open(my $fh4, '<', $0);  # Prints Ho!

BEGIN {
        delete $main::{open};
}

open(my $fh5, '<', $0);  # Calls CORE::open

__END__
Hi
Ho