use Regexp::Common;

   while( $html =~ s/($RE{balanced}{-parens=>'<>'})// )
   {
      $tags{$-[0]} .= $1;
   }

   $html =~ s/foo/bar/g;

   foreach my $point (sort {$b<=>$a} keys (%tags))
   {
      substr($html, $point, 0 ) = $tags{$point};
   }