Generally using regexen to extract data from HTML is hard enough that it is worth leaving to tools designed for the purpose. In this case HTML::TreeBuilder is one way to do the job:

use strict;
use warnings;
use HTML::TreeBuilder;

my $html = <<HTML;
<p><a id="GuidelineDataList__ctl1_doctitlelink"
href="/summary/summary.aspx?doc_id=4363">(1) Pertussis vaccination: us
+e of
acellular pertussis vaccines among infants and young children. <br />(
+2) Use of
diphtheria toxoid-tetanus toxoid-acellular pertussis vaccine as a five
+-dose
series. (Addendum)</a>
  Centers for Disease Control and Prevention - Federal Government Agen
+cy [U.S.].
  1997 Mar 28 (revised 2000 Nov; addendum released 2003 Sep 26)
.
  25 pages.
 
  NGC:003288</p>
HTML

my $tree = HTML::TreeBuilder->new_from_content ($html);

# Get a list of anchors that have an id attribute starting 'GuidelineD
+ataList'
my @anchors = $tree->look_down (id => qr/^GuidelineDataList/);

for my $anchor (@anchors) {
  next if ! defined $anchor || ! defined $anchor->parent ();
  print 'HRef: ', $anchor->attr ('href'), "\n" if defined $anchor->att
+r ('href');
  print 'Text: ', $anchor->parent ()->as_text (), "\n" if defined $anc
+hor->parent ();
}
[download]

Prints:

HRef: /summary/summary.aspx?doc_id=4363
Text: (1) Pertussis vaccination: use of acellular pertussis vaccines a
+mong infants and young children. (2) Use of diphtheria toxoid-tetanus
+ toxoid-acellular pertussis vaccine as a five-dose series. (Addendum)
+ Centers for Disease Control and Prevention - Federal Government Agen
+cy [U.S.]. 1997 Mar 28 (revised 2000 Nov; addendum released 2003 Sep 
+26) . 25 pages. NGC:003288
[download]