my $sth=$dbh->prepare("select ....");
$sth->execute;

# Stuff all the data in a hash, concatenating all comments
my %Macs;
while (my $hr = $sth->fetchrow_hashref) {
    if (! exists $Macs{$$hr{mac}}) {
        $Macs{$$hr{mac}} = $$hr{comment};
    }
    else {
        $Macs{$$hr{mac}} .= $$hr{comment};
    }
}