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 {
        # Deleting and remaking the sub fails! ;(
        delete $main::{open};
        *{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__
Ambiguous use of *{main::open} resolved to *main::open at examples/black-magic.pl line 17.
Subroutine main::open redefined at examples/black-magic.pl line 17.
Ho