Somehow, my contribution to this was lost. I quietly suspect the fault is sitting in front of the keyboard and it's all related to the preview-button.

Hence I write this again. This solution has the added benefit of trivially supporting partitions other than two per sublist. Autovivification++!

my @a = (1..21);
my @b;
push @{$b[$_/2]}, $a[$_] for 0..$#a;
[download]

Cheers,
Steffen