$_ = <<'HTML';
<a href="http://www.foobar.com/xxx">
<a href="http://www.danheller.com/xxx">
<a href="http://www.foobar.com/xxx">
<a href="http://www.danheller.com/xxx">
HTML

s{(href="http://(?!www\.danheller\.com)[^"]+")}{$1 target="_blank"}g;

print;

##</code><code>##

<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">