[2024] what exactly does the 'pack mule'-perk do ?

When you're looking at your inventory, you see how there are 3 rows of gray squares, and then 2 rows of what look like locked slots? Pack mule 'unlocks' those 'locked' slots making them into the same gray squares as the upper three rows.

You get slowed down when your total number of unique slot occupations exceeds your number of gray squares, regardless of where the items are positioned in the inventory. If you have 30 gray squares and 31 gray items (even if half of those items are in the 'locked' slots), then you will be slowed down.

In short, Pack Mule gives you 3 more inventory slots to fill before you would be slowed down.