Parse::RecDescent is used to create parsers, yet there already exists a parser for URIs. URI and extention URI::QueryParam should do the trick.

Update: Here's an example:

use URI             qw( );
use URI::QueryParam qw( );

foreach (
   'http://www.perlmonks.org/index.pl?node_id=629153',
   'http://www.perlmonks.org/index.pl?node=Recently%20Active%20Threads
+',
) {
   my $uri = URI->new($_);
   my @node_ids    = $uri->query_param('node_id');
   my @node_titles = $uri->query_param('node');

   if (  (@node_ids && @node_titles)
      || @node_ids    > 2
      || @node_titles > 2
   ) {
      warn("$uri: Error: Bad uri\n");
   }

   if (!@node_ids && !@node_titles) {
      warn("$uri: Warning: Unrecognized uri\n");
      next;
   }

   if (@node_ids) {
      print("$uri: By Id ($node_ids[0])\n");
   }

   if (@node_titles) {
      print("$uri: By Title ($node_titles[0])\n");
   }
}
[download]

Or maybe you are trying to extract data from a download HTML page? If so, use an existing HTML parser (such as HTML::TreeBuilder and HTML::Tree) instead of rolling out your own.

I've found XPath to be very useful. HTML::TreeBuilder::XPath allows you to query the HTML document for information. The Firebug extention for Firefox can help you find the paths.

If PerlMonks is not just an example, I recommend download the XML version of pages by adding the displaytype=xml query parameter to requested URIs. The same advice I gave for HTML applies for XML. Use an existing parser, and XPath is very useful for XML too.