While the prior replies are very good general answers, if you wanted to directly find the size of just the second array in the AoA @A you could use  scalar(@{$a[1]}) where $a[1] is the reference to the second list and  @{$a[1]} deferences that to an array and  scalar(@{$a[1]}) takes the size of that.

edit: opps i should have replied to the OP and not a reply, sorry

2018-05-16 Athanasius reparented to OP