# first, get all the results for the record set used in inner loop
my $record_set_1 = $sth1->fetchall_arrayref;

# loop over second record set
while (my @row = $sth2->fetchrow_array)
{
    foreach my $row_from_record_set_1 (@$record_set_1)
    {
        # blah blah
    }
}