Yuk. Just return the extracted data from your parse rule. What follows is a simpler example.

To build your parser module:

#!/usr/bin/env perl

# make_grammar.pl

use strict;
use warnings;

use Parse::RecDescent qw( );

my $grammar = <<'__END_OF_GRAMMAR__';

    {
        use strict;
        use warnings;
    }

    parse : list /\Z/ { $item[1] }

    list  : '(' term(s) ')' { $item[2] }

    term  : /[a-z]+/
          | list

__END_OF_GRAMMAR__

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

To load the file, you'd do something like:

use NestedListParser qw( );

sub load {
    my $self = shift;
    my $qfn  = shift;

    open(my $fh, '<', $qfn)
        or croak("Couldn't open file \"$qfn\": $!");

    my $file; { local $/; $file = <$fh>; }

    my $parser = NestedListParser->new();
    $self->{list} = $parser->parse($file)
        or die("Bad data\n");
}
[download]

Updated: Changed to a grammar that better illustrates the solution.