It seems that you can define "top level" actions and define variables in them. So this would do the trick:

use strict;
use warnings;
use Data::Dumper
use Parse::RecDescent;

### use vars qw/@tokens/;  Avoided using global variables
my $p6 = Parse::RecDescent->new(q(
                                 {my @tokens}
                                 letter : /\w/
                                 character : letter
                                 {
                                   push @tokens, $item{letter};
                                 }
                                 Format : character(s) { \@tokens }
));
my $tokens_ref = $p6->Format('abc');
die "Invalid pattern" unless (defined $tokens_ref);
print Dumper $tokens_ref;
[download]

Outputs:

$VAR1 = [
          'a',
          'b',
          'c'
        ];
[download]