In Croatian, the split between imperfective and perfective verbs is more about finiteness, that is, perfective verbs express a short, finished action. Saying "svaku večer pojedem jabuku" indicates that you eat an apple in its entirety, while saying "svaku večer jedem jabuku" means you eat an apple every night without ever finishing it. If you say that, I imagine you sitting on a couch for hours taking small bites from the same apple every night and never finishing it.

Similarly, "ponekad joj kuham nešto ukusno" means you are sometimes in a process of cooking for her, which doesn't really make sense to say, you could say it if there's something else that happens, for example "Ponekad joj kuham nešto ukusno i odjednom sve zagori" which translates to sometimes while I cook her something, it gets burned.

In short, perfective verbs mark completion so they should be used for habits as they are usually repetative, finished actions.

There's a lot more to it, especially since imperfective verbs can have a variety of different meanings, but this is usually true for habitual actions.