My understanding is that venial sin is always a deliberate action or thought against God’s laws, or failure to do something that is good that you really could (and probably should!) do.
I believe that, if an evil action is not deliberate then it is not venial sin. In that case it is an imperfection. Right?
So for example, lets say at one time in my life I swore like a sailor all day everyday, then by the grace of God I learned to stop swearing. And years after stopping swearing, one day I got very angry and out popped an f bomb simply out of “old habits die hard” and carelessness in anger. That is an imperfection.
But then, in my anger a couple more swears pop out, but then a couple days later, when I want to swear, I think about the bad word I want to say and tell myself not to swear…but then I swear anyways, probably because I haven’t been doing to much to stop the swearing out of anger in the past few days and so I’m now weak in that area. That is a venial sin.