You really just need to look at the source for DBI, version 1.14 (I think that's when this feature was added). It's not documented, as the Todo points out:

    document dbi_fetchall_arrayref_attr attr of selectall_arrayref().
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Anyway, the way this works is that this attribute controls the behavior of selectall_arrayref. If you don't set it, it defaults to an arrayref, which tells the method to fetch the rows as array refs and return an arrayref of those arrayrefs.

If, however, you set it to a hash ref, that tells selectall_arrayref to fetch the rows as hash refs, and return an array ref of hash refs. Make sense?