Can't you just build your grammar up from pieces? e.g.:

my $expression_grammar =
    qq(# Your common grammar here
       );

my $facts_grammar =
    qq(main:
           ....
        $expression_grammar
       );

my $rules_grammar =
    qq(main:
           ...
       $expression_grammar
       );


my $facts_parser = Parse::RecDescent->new($facts_grammar);
my $rules_parser = Parse::RecDescent->new($rules_grammar);
[download]

-ben