You are indeed correct that the classic solution to the problem is a graph and the type of sort is topological. However, the perl code is not compelled to support every step of the theory. It turns out to be much simpler in practice with perl sort:

    if $a is dependent on $b, return -1
    if the reverse, return 1
    otherwise return 0
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this lets $a and $b remain unsorted relative to each other where there is no dependency.