You description doesn't tell that the elements are shuffled the way your example code says.

if consecutive elements and destroying the original array are OK, try splice

  DB<32> use Data::Dump qw/dd/

  DB<33> @a=1..12; dd { map { $_ => [splice @a,0,3] } 1..4  }
{ 1 => [1, 2, 3], 2 => [4, 5, 6], 3 => [7, 8, 9], 4 => [10, 11, 12] } 
+...
[download]

NB: the case where @a/4 is not an integer is more complicated.

HTH! :)