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

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

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

BEGIN {
        # Deleting and remaking the sub fails! ;(
...
                import subs 'open';
        }
}

use strict;
use warnings;
...
}

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