my @row = $_read_sth->fetchall_arrayref

That should ring a bell - the method returns an arrayref, and you're assigning it to an array. Not to an array reference.

BTW if you need to process the data row by row, why don't you just read the data row by row from the db? That's far more efficient (especially in terms of memory)

while (my @row = $sth->fetchrow_array){
   print join(", ", @row), "\n";
}
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