The following seems to work: 
$_ = join '', <DATA>;
s/(<a href="https?:\/\/(?:(?!target=).)*?)>/$1 target="_blank">/igs;
print;

__DATA__
<a href="dsfsdf">sdfdsf</a>
<a href="http://dsfsdf">sdfdsf</a>
<a href="https://dsfsdf">sdfdsf</a>
<a href="http://dsfsdf" target="_blank">sdfdsf</a>
<a href="http://dsfsdf"
>sdfdsf</a>

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In reply to Re: Regexp: Match anything except a certain word by TedPride
in thread Regexp: Match anything except a certain word by fraktalisman

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