use DBI;

my ($db, $user, $pass, $dbh, $query, $sth, $record, %clients);

$db = 'database';
$user = 'user';
$pass = 'password';

### Log into database
die "Couldn't connect to database: " . DBI->errstr
    if !($dbh = DBI->connect("DBI:mysql:$db", $user, $pass));

### Select client list
$query = 'SELECT * FROM clients';
$sth = $dbh->prepare($query);
$sth->execute();    

### Store by record IDN or other identifying field
$clients{$record->{'idn'}} = $record
    while $record = $sth->fetchrow_hashref();

### Add data from different table
$query = 'SELECT * FROM client_data';
$sth = $dbh->prepare($query);
$sth->execute();    

### Index on same IDN
while ($record = $sth->fetchrow_hashref()) {
    push @{$clients{$record->{'client'}}{$_}}, $record
        for qw/client data fields here/;
}

### Sort on some field to return to desired order
for $record (sort { $a->{'name'} cmp $b->{'name'} } values %clients) {
    ### Do something with record
}