Great question. I guess when we think about perception in the context of this great collection of poems, we are struck most of all by the way that Blake presents us with a whole series of poems that are paired together, but offer radically different perceptions of the same topic, such as "The Chimney Sweeper." When we understand that Blake offers us the same subject but viewed from the position of innocence and then experience, we understand a bit more about what he is trying to achieve.
In his songs of "innocence," Blake offers us a perception based on a condition essentially allied to childhood, a condition in which we can view the world without fear and can feel confident that there is a home for us. However, as the songs of "experience" make evident, innocence is unable to exist for long without being challenged. In these songs giving a radically different view on life, Blake presents us with a terrible world, where the various social evils are made much worse through all kinds of tyranny and harshness that is political, religious and personal.
Thus when we consider perception, perhaps we need to consider that Blake is trying to argue for a middle ground, somewhere between the two opposing states of innocence and experience, that allow us to maintain something of the child's view of the world, but at the same time does not make us naive about the various evils and worries of the world. This collection of poems above all challenges us and our way of viewing the world.