The trick is to decide what you need from %ENV and either detaint it or set it explicitly. The perlsec pod tells about how to do that.

Make sure to do it in a BEGIN block before loading Env.pm.

Update: Whoops! I had taint on the mind, this doesn't answer your question (though I insist it's good to know :-). As jettero says, our is the answer, or use vars ($FOO, $BAR);