That won't work because you're not importing the version of open() defined within open_temp.pm into package main. You really should just read the perlsub document, like I linked to before. And honestly, I don't think you've followed my advice about really really really thinking about whether you actually need to do it this way. Just because Perl lets you override built-in functions doesn't mean you should. Why do you need an overridden open function? Why not just call your new function "myopen()"? Do you have to override for some good reason?