modifying an element in an element

$letterx = (
"xx__xx__x",
"xx\ \/ /x",
"xxx>  <xx",
"xx/_/\_\x",
"xxxxxxxxx")

$box = (
"         ",
"         ",
"         ",
"         ",
"         ")

I would like to replace the 3rd element of the first element of $letterx to the 3rd element of the first element of $box

this faulty code gives you the idea of what I am trying to do, but it is not proper

$box[0][2] = $letterx[0][2]

Any suggestions on how to accomplish this?