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$_ = <<'HTML'; <a href="http://www.foobar.com/xxx"> ... s{(href="http://(?!www\.danheller\.com)[^"]+")}{$1 target="_blank"}g; print;
<a href="http://www.foobar.com/xxx" target="_blank"> <a href="http://www.danheller.com/xxx"> <a href="http://www.foobar.com/xxx" target="_blank"> <a href="http://www.danheller.com/xxx">