my $parse = Parse::RecDescent->new(<<'EndGrammar');

rebol   : block  { dump_item('block', \@item)  }
        | scalar { dump_item('scalar', \@item) }

block       : '[' block_stuff(s?) ']'
block_stuff : scalar
scalar      : <skip:''> '%' file
file        : /w+/

EndGrammar