When the score is settled, each character of the text can only be matched by two rules if one rule is a production of another.

text:      struct  {   int   foo   ;   int   bar   ;   }
           ------  -   ---   ---   -   ---   ---   -   -
           IDENT  "{" IDENT IDENT ";" IDENT IDENT ";" "}"
                       ---   ---       ---   ---
                       type  var       type  var
                       ---------       ---------
                         decl            decl
                       -------------------------
                              decl_list
           ---------------------------------------------
                              struct
           ---------------------------------------------
                              parse
[download]

But in reaching that state, a rule can match, then be unmatched by a backtrack. For example, given the grammar

parse : foo1 foo2
      | bar1 bar2
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foo1 could matched, but PRD will backtrack if it can't follow with a foo2 match. It will then try bar1.

I'm guessing one of your productions has side-effects, so you falsely believed it has matched even though a backtrack unmatched it. I could very well be wrong because I have very little data to go on.

Update: In the following example, you'll see foo1 on the screen even though it wasn't matched.

use strict;
use warnings;

use Parse::RecDescent qw( );

my $grammar = <<'__EOI__';
   {
      use strict;
      use warnings;
   }

   parse : foo1 foo2 /\Z/ { [ @item[0,1,2] ] }
         | bar1 bar2 /\Z/ { [ @item[0,1,2] ] }

   foo1  : "X" { print("$item[0]\n"); [ @item[0,1] ] }
   foo2  : "Y" { print("$item[0]\n"); [ @item[0,1] ] }
   bar1  : "X" { print("$item[0]\n"); [ @item[0,1] ] }
   bar2  : "Z" { print("$item[0]\n"); [ @item[0,1] ] }

__EOI__

Parse::RecDescent->Precompile($grammar, 'Grammar')
   or die("Bad grammar\n");
[download]

use strict;
use warnings;

use Data::Dumper qw( Dumper );
use Grammar      qw( );

my $parser = Grammar->new();

my $matches = $parser->parse('XZ')
   or die("Bad input\n");

print("\n");

print(Dumper($matches));
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foo1
bar1
bar2

$VAR1 = [
          'parse',
          [
            'bar1',
            'X'
          ],
          [
            'bar2',
            'Z'
          ]
        ];
[download]