Location - Go down to the bio lab and find where the zombies are trapped in the cells behind the blue cooling towers with bodies in them.
Cages - Once you have found the trapped zombies kill all the zombies in those cages. Now advance one round. The cages should now be filled with Vermins. Kill all the Vermins and advance another round. Now when the round has advanced a mangler will be in the cages which you have to kill. Once it it killed free perk should spawn where you are standing.