start: tree
tree: '(' treestr(s) ')'
treestr: tree | tagstr
tagstr: TAG ( tree | word )
TAG: /[A-Z.]+ /
word: /[\w?]+/

##</code><code>##

#!/usr/bin/perl

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

$::RD_AUTOACTION = q { [@item] };
# $::RD_HINT = 1;

my $grammar= q {
start: tree
tree: '(' treestr(s) ')'
treestr: tree | tagstr
tagstr: TAG ( tree | word )
TAG: /[A-Z.]+ /
word: /[\w?!.]+/
};
my $parser=Parse::RecDescent->new($grammar);
my $text = "(SBARQ (WHNP (WP What))(SQ (VBZ is)(NP (NNP Head)(NNP Start)))(. ?))";
my $result = $parser->start( \$text );
print $text, "\n";
print Dumper($result);