my @tokens = get_tokens $string;

my sub parse_list {
   my @list;
   while ( @tokens && $tokens[0] ne ")" ) {
      my $token = shift( @tokens );
      if ( $token eq "(" ) {
         push @list, __SUB__->();
         die( "Missing `)`" ) if !@token;
         my $token = shift( @tokens );
         die( "Missing `)`" ) if $token ne ")";
      }  else {
         push @list, $token;
      }
   }

   return \@list;
};

my $list = parse_list();
die( "Unexpected `$tokens[0]`" ) if @tokens;