Hi madbee,
There are several ways you can use to make this happen. Though following your "outline" could make it make ( I didn't try it), but using regex could also do this:

use warnings;
use strict;

while(<DATA>){
  if(/controlled/i){
   if(/(?<=a)\s(.+?)-?(?=controlled)/i){ #NOTE HERE
    print $1,$/;
   }
  }
}

__DATA__
This is a design-controlled string
This is a design controlled string
[download]

..produces...

design
design
[download]

NOTE: This might not be a generic solution, simply because am using OP data.
Please see Using Look-ahead and Look-behind