my $code = "( Tab1 ( TabA TabB ) Tab2 Tab3 )";
my @stack = split / /, $code;

print Combine(\@stack);

sub Combine {
   my $stack = shift;
   my $retval = '';

   my @elements = ();   
   while(scalar @{$stack}) {
      my $element = pop @{$stack};
      if($element eq ')') {
         push @elements, '[' . Combine($stack) . ']';
      } elsif($element eq '(') {
        last;      
      } else {
         push @elements, $element;
      }
   }

   $retval .= join("#", reverse @elements);
   return $retval;
}