Some of us may be -- uh, challenged -- (well, /me certainly is) by the lack of an explicit declaration that sect_num, eg, 4, is the z value... or that z has some other value...OR that it's the mystery-value you're referring to with the phrase "partial data." Then too, the [seq], in your problem statement, has an undefined value (Halp! Where's it come from?)

All that whining aside, why not an AoA?

my @AoA = (["seq1", 1, 1, 4],
           ["seq2", 2, 1, 4],
           ["seq3", 2, 2, 4],
           ["seq4", 1, 2, 4],
          );
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