# alte Variante

$txt =~ s%(\s)(www\.[^\s<]*)%$1<a href="http://$2" target="_blank">$2</a>%sg;

# andere (nicht funktionierende) Variante

  $txt =~ s%\w(www\.[^\s<]*)%<a href="http://$1" target="_blank">$1</a>%sg;