*{main::open} = sub { print "Ho\n"; };

##</code><code>##

my $glob_ref = do { no strict 'refs'; \*{'main::open'} };
*$glob_ref = sub { print "Ho\n"; };

##</code><code>##

BEGIN {
        # Deleting and remaking the sub fails! ;(
        delete $main::{open};
        *{main::open} = sub { print "Ho\n"; };
        use subs 'open';
}

##</code><code>##

BEGIN {
        # Deleting and remaking the sub fails! ;(
        delete $main::{open};
        *{main::open} = sub { print "Ho\n"; };
        BEGIN {
                require subs;
                import subs 'open';
        }
}

##</code><code>##

use strict;
use warnings;

use subs qw( );

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

BEGIN {
        my $glob = do { no strict 'refs'; \*{'main::open'} };
        *$glob = sub { print "Hi\n"; };
        import subs 'open';
}

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

BEGIN {
        delete $main::{open};

        my $glob = do { no strict 'refs'; \*{'main::open'} };
        *$glob = sub { print "Ho\n"; };
        import subs 'open';
}

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

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

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