The elemental pierce perk is a perk that makes the creature more suscetible to the element. The conversion makes 1% of elemental pierce to 1% more damage if the element resistance is below 100% or 1% of elemental pierce to 0,5% more damage if the element resistance is 100% or more.
Note that the elemental pierce does not makes the weapon hit that element, so you'll need to imbue with elemental conversion damage in order to make it function. That said, take a look into some examples below:
If you are using a crypt slicer with the 3 earth pierce (45% earth pierce) perks against a Dragon Lord, who takes only 20% earth damage, while using a Crypt Slicer imbued with Earth damage, the earth damage taken will be 65% instead of 20%.
On the other side, for creatures that are already weak to earth damage, like Weretigers (115%), the 45% earth pierce will make him 22,5% more vulnerable (137,5%) against this damage.
If you are facing an Harpy, who takes 90% earth damage, the earth pierce will first upgrade the damage will go to 117,5% (10% + [(45%-10%)/2].
