in reply to replacing chunks of text only if another chunk matches a pattern

$_ = <<'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;

[download]

Results: 

<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">

[download]

In Section Seekers of Perl Wisdom