use vars qw($animal);
use Parse::RecDescent;

$grammar = q {
            object:     thing[article=>'a'](s)
              |     thing[article=>'the'](s)

            thing:     <matchrule:$arg{article}>
                   <matchrule:${\scalar reverse $::animal}>

            a:        'a'
            the:    'the'

            cat:    'cat' { print "meow\n"; $::animal = 'god' }
            dog:    'dog' { print "woof\n" }
         };

unless( $parser = new Parse::RecDescent( $grammar ))
{
    die "bad grammar; bailing\n";
}

$/ = "";
while (defined ($input = <DATA>))
{
    $::animal = reverse 'cat';

    print STDERR "parsing...\n";
    unless( defined $parser->object( $input ))
    {
        die "error in input; bailing\n";
    }
    print STDERR "...parsed\n";
}

__DATA__
a cat
a cat
a cat
a dog

the cat
the dog
the dog
the dog
the dog
a dog