This is the Luck 5 perk:

Dumb Luck

Randomly receive triple XP for any action. The lower your Intelligence, the greater the chance.

This was translated from Spanish, so I don't have the exact wording, but there it is.

Would be great if true, that way we aren't shoehorned into taking INT if we want more EXP.

Certainly makes sense from the look of the perk. Vault Boy looked stoned or not completely there in that picture.

Do you have a link that has them all?

here you go:

http://orcz.com/Fallout_4:_Perk_Chart

Finally. It makes sense now. It's also very useful.

So technically, upon completion of a major quest and thus receiving a fairly sizable amount of XP, one could save beforehand and reload until your awarded triple XP.

If one was so inclined.

It may be just based on XP from simple actions like killing an enemy or picking a lock, or perhaps yes, one could save-scum a quest end to try for the bonus XP. If one was so inclined.

Now all I need to know is what Luck 9 does

lol, makes sense.