As Parse::RecDescent gets its grammar as a string, I see no way to create clever closures or anything that will allow you to use lexical variables. One approach I can think of would be judicious use of local to isolate the effects, but that implies that your parsers won't call each other recursively:

...
sub do_parse {
    local $parser_info = {
        tokens => [],
    };
    my $p6 = Parse::RecDescent->new(q{
        {
            push @{ $parser_info->{tokens} }, $item{letter}
        }
    });
};
...
[download]

That way, you reduce your need for a global variable to one entry point, $parser_info. I think you can also instruct Parse::RecDescent to return you the whole parse tree as an AST instead of having it execute code immediately, by using the <autotree> directive. That wouldn't require any code within your parser, but you have to walk the tree yourself after it has been constructed.

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]

citromatik

my $p6 = Parse::RecDescent->new(q(
                                 letter : /\w/
                                 character : letter
                              
                                 Format : character(s)
));
my $x= $p6->Format('abc');
die "Invalid pattern" unless (defined $p6);
print Dumper $x;

#prints

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

Be careful to specify what a rule returns with $return if more than one production is in a rule (i.e. with a rule like "strangeword: character number character(s)", since only the last production is returned as a default

With $return you can also return anything you want, i.e. parsed and interpreted commands, hashes, objects ...