The following is based on Fallout 3 cut content and has not been confirmed by canon sources.
At some point during development the Ghoul Ecology perk was meant to be obtained by reading a book titled "10 Indisputable Rules On Ghouls". The book is still present in the DLC's code, and if somehow obtained will grant the perk on use. For PC users, this can be accomplished by using the command player.additem xx00fc30 1.
End of information based on Fallout 3 cut content.
After receiving this perk, weapons will show with the damage increase in their Pip-Boy. Rapid-fire weapons such as the Vengeance or the Xuanlong assault rifle show very large increases in the DAM stat as the +5 damage bonus is applied per shot.
Due to a misplaced condition, damage is applied to all enemies. The condition is placed into the conditions list used for perks that are selectable at level-up, instead of in the Perk Entry's Target condition where it belongs. This means that the damage bonus is not just restricted to ghouls like the Entomologist perk is against insects, but instead the damage is added against EVERY enemy you encounter.
The bonus is applied on each shot fired in V.A.T.S. Therefore, hunting rifles (which fire once) get +5 and Chinese assault rifles (which shoot 3 times at once) get a bonus of +15. This can be useful for players who use weapons that fire many shots in V.A.T.S., such as the minigun, which fires 8 times providing a bonus of +40 damage.