This may not be exactly what you want but it is probabbly a little closer. I added the numbers after the tags to track which open/close pair I was looking at. Left 'em in.

                                                                      
+                                                      
use strict;
use Parse::RecDescent;

my $parser = Parse::RecDescent->new(q(
                                    start: part(s)
                                          {
                                             print join("\n",@{$item[1
+]});
                                          }

                                      part : stag part(s) etag
                                             {
                                                 $return = "$item{stag
+} " . join(" ", @{$item[2]}) . " $item{etag}";
                                             }
                                             | stag data etag
                                             {
                                                 $return = "$item{stag
+} $item{data} $item{etag}";
                                             }
                                             | data



                                      data  : m{([^<]+)}

                                      stag : m{<open\d>}
                                      etag : m{<close\d>}
));

$parser->start(<DATA>);
__DATA__
<open1> Some content <close1> <open2> some <open3> more content <close
+3> <close2>
[download]

gives the following output

<open1> Some content  <close1>
<open2> some  <open3> more content  <close3> <close2>
[download]

The formatting of the second clause can be twekaed with the join char.
I am by no means a P::RD master, but I really enjoy playing with it. Let me know if this helps.