From the documentation that you linked to, calling partitions( \@data, 3, 4 ) is not a supported usage. Reading the code linked from there, it appears to be the same as calling partitions( \@data, 3 ) which would produce only partitions composed of exactly 3 subsets, not partitions of any size but with no more than 3 members in each subset of the partition (the latter being what, to me, the OP seems to have asked for).