The rightmost line of perks in the Enchanting constellation terminates with a perk called Soul Siphon, which causes your deathblows on creatures (but NOT people) to trap 5% of the victim. What does this mean? Does this require you to have an unused soul gem in inventory for it to work, and how does it interact with a soul trap weapon?
It means, you can take out your completely empty Sword of Flaming Flames, kill a bear with it, and now it's got 5% of the bear's soul for the next fight. (note: works for partially filled magic items too)
Yes, that is 105% of the soul, which is awesome.
That description seems off, or at least misleading. The full perk description is:
The Soul Siphon perk isn't meant to trap souls in the normal sense (like you would with Soul Trap), it's meant to be an automatic recharge for your enchanted weapon.
Skizzlefrits mentions in the comments how it works: