use strict; 
use warnings; 
use Parse::RecDescent;
use File::Slurp;

my $grammar = 'startrule: ( "a" | "aa" ) "a"';
my $parser  = Parse::RecDescent->new($grammar);

my $text = read_file('input.txt');
write_file('output.txt', $parser->startrule($text), "\n");