You are not copying an array into your array. You are putting in the result of the operation
7,2,3 which is 7 @one = 7,2,3 which due to precedence is evaluated as (my @one = 7),2,3 as previously explained in this thread. To get the array I think you want in there:
my @one = (7,2,3);
7 2 3
Pereant, qui ante nos nostra dixerunt!
shmem pointed out my sloppy summary of this problem. I was concentrating on what I thought he wanted to achieve and lost sight of the true explanation.