use Parse::RecDescent;
use Data::Dumper;

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

my $grammar= q {

start   :seq

seq	: '('TAG (seq | word) | ("." "?")')'

TAG     : /[A-Z]+/

word   : m(\w*)

 
};
my $parser=Parse::RecDescent->new($grammar);
my $result = $parser->start("(SBARQ (WHNP (WP What))(SQ (VBZ is)(NP (NNP Head)(NNP Start)))(. ?))");
print Dumper($result);