Each successive pass of walktree clobbers the last results of fetchrow_array, since it executes on the same statement handle. One solution is to fetch all the rows at once, like this:

    my ($subid, @children);
    while (($subid) = $children->fetchrow_array) {
        push @children, $subid;
    }
    for $subid (@children) {
        walktree ($subid);
    }
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instead of:

    while(my ($subid) = $children->fetchrow_array) {
        walktree($subid);
    }
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