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BEGIN {
    *CORE::GLOBAL::open = sub (*;$$) { return undef };
}
open(my $fh, '>', 'f.tmp') or die "error: open";
print $fh "hello\n" or die "error: print";

BEGIN {
    *CORE::GLOBAL::print = sub (*;@) { return undef };
}
open(my $fh, '>', 'f.tmp') or die "error: open";
print $fh "hello\n" or die "error: print";