The target="_top" attribute has to be given on the original anchor or form tag that initiated the request to your CGI. You cannot, unfortunately, specify a target in a redirect header.

It might be possible to create a hack where, instead of returning an actual redirect header, you return a page that has a zero second meta-refresh tag with a target="_top" attribute but that relies on their browser supporting meta-refresh tags, which not all do. And it's still a hack. *shrug*

bbfu
Seasons don't fear The Reaper.
Nor do the wind, the sun, and the rain.
We can be like they are.