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 my $hoh = $dbh->selectall_hashref($sql, $index_column);

$hoh = {
    1 => {id => 1, col2=>"abc", col3=>"def",    col4=>"ghi"},
    2 => {id => 2, col2=>"lmn", col3=>"opq",    col4=>"rst"},
}

 my $aoa = $dbh->selectall_arrayrefref($sql);

$aoa = [
    [1,"abc", "def", "ghi"],
    [2,"lmn", "opq", "rst"],
]

 my $aoh = $dbh->selectall_arrayref($sql, {Slice => {}});

$aoh = [
    {id => 1, col2=>"abc", col3=>"def",    col4=>"ghi"},
    {id => 2, col2=>"lmn", col3=>"opq",    col4=>"rst"},
]