He wasn't fetching using fetchrow_arrayref or fetchrow_hashref he was fetching with selectall_hashref and it does make a difference:

SQL> create table mje(mac varchar(10), comment varchar(10));
SQLRowCount returns -1
SQL> insert into mje values('1226', 'comment1');
SQLRowCount returns 1
SQL> insert into mje values('1226', 'comment2');
SQLRowCount returns 1
SQL> insert into mje values('1226', 'comment3');
SQLRowCount returns 1
SQL> select * from mje;
+-----------+-----------+
| mac       | comment   |
+-----------+-----------+
| 1226      | comment1  |
| 1226      | comment2  |
| 1226      | comment3  |
+-----------+-----------+
SQLRowCount returns -1
3 rows fetched
SQL>
[download]

perl -le 'use DBI; my $h = DBI->connect;nect; my $r = $h->selectall_ha
+shref(q/select * from mje/, "mac"); use Data::Dumper; print Dumper($r
+);'
$VAR1 = {
          '1226' => {
                      'comment' => 'comment3',
                      'mac' => '1226'
                    }
        };
[download]

perl -le 'use DBI; my $h = DBI->connect;nect; my $r = $h->selectall_ar
+rayref(q/select * from mje/); use Data::Dumper; print Dumper($r);'
$VAR1 = [
          [
            '1226',
            'comment1'
          ],
          [
            '1226',
            'comment2'
          ],
          [
            '1226',
            'comment3'
          ]
        ];
[download]

If you use selectall_hashref or fetchall_hashref you need to provide a key column and if you have multiple rows where that key column has the same value you will lose some rows.