Well spotted! Right now I cannot think of something as terse as this for the general solution. The $i&4 basically tells the ups and downs and would need to be replaced by something like int($i/@K)&1 (not tested). The line based on sin will not work for large arrays @K as sin is not linear enough for that...

Update: This should work:

push @{ $hash{ $K[ int(++$i/@K)&1 ? @K-1-$i%@K : $i%@K ] } }, $_ for @
+V;
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