I agree. If you are doing a redirect anyways, there can hardly be any reasons not to go for "_parent" right away. If it is a form post, form also has the attribute "target", so just do as you would any link. The drawback being that it will send all posts from that form to that frame (see below).

If you want to conditionally open in the same frame or in the top frame depending on what was posted, or which button was pressed in a form, then you are pretty much out of luck, if you want to be cross-platform. You can do this easily enough with onSubmit javascripts - if all your users have javascript, and has it enabled. Or you can try using non-standard headers like you do now. None of which will ever work reliably, and unless it is some inhouse production where you can control the end users browser, I'd suggest a rethinking of your design.

Like Ovid below, I am also very curious as to the "can't" use CGI.pm. Does that mean you don't know how, or is it possible that there (still) exists an installation where CGI.pm isn't available?

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You have moved into a dark place.
It is pitch black. You are likely to be eaten by a grue.