For safety's sake, I urge against trying to convince all but the most serene DM that keeping a cloak of charisma in a backpack still enables the possessor to gain its benefit. Dice can hurt when thrown hard enough. That said...
It's possible (but unlikely) that the designers intended a cloak of charisma to be kept in a creature's possession instead of actually worn...
I've not heard anything from Monte Cook et al. about this (and such industry figures are unlikely to respond to my queries anyway), but the Dungeon Master's Guide on Using Items says
Continually functioning items, such as a cloak of resistance or a headband of intellect, are practically always items that one wears. A few, such as a pearl of power, must simply be in the character’s possession (on his person, not at home in a locked trunk). (213)
Emphasis mine. An argument can be made, then, that picking as an example the cloak of resistance when the cloak of charisma is the obvious alphabetical choice speaks to designer intent: after reading (or even writing!) the passage quoted above, the designer of the cloak of charisma—such an argument would go—seems to have wanted to exclude a creature from needing to wear a cloak of charisma to realize its benefit and carefully phrased the description so that such a cloak could be shoved into a haversack and its possessor could wear instead, for example, a cloak of resistance.1,2
...But it's strongly implied that any magic cloak be worn to gain its benefits
The Dungeon Master's Guide on Magic Items on the Body says
Many magic items need to be donned by a character who wants to employ them or benefit from their abilities. [...E]each of those items must be worn on (or over) a particular part of the body. A humanoid-shaped body can be decked out in magic gear consisting of one item from each of the following groups, keyed to which place on the body the item is worn.[...]
- One cloak, cape, or mantle around the shoulders (over a robe or suit of armor) (214)
So while an argument can be made for still gaining an enhancement bonus from a cloak of charisma +2 folded up in a backpack, that's not how a magic cloak is employed by one who wants to benefit from its abilities. A cloak of charisma is not a chaos diamond or a stone of controlling earth elementals or even a luck blade—all of which require possession but having types that aren't listed on that chart. The cloak of charisma is a specifically noted type of magical item that occupies a body slot, a cloak.
1 Maybe that designer was playing a sorcerer?
2 However, truly first alphabetically among the Dungeon Master's Guide's wondrous items is the amulet of health, and that item needs to be worn, so make of that argument what you will.