Near as I can tell you are hoping to create a header for each file, then populate the auto line, then populate the rest of the file. However that is a rather fragile concatenation of circumstances! There are two approaches that are likely to work better for you: modify your select so you get the host rows together (my $sth = $dbh->prepare("SELECT host, name, type, address, netmask, gateway FROM $table_name order by host");), or make a preliminary pass over the data, then create the files in a second pass. The second approach is shown first:

use strict;
use warnings;

my @data = (
    ['app12', 'eth1', 'static', '192.168.0.5', '255.255.255.0', '192.1
+68.0.1'],
    ['app15', 'eth0', 'dhcp',   '',            '',              ''],
    ['app15', 'eth1', 'static', '192.168.0.4', '255.255.255.0', '192.1
+68.0.1'],
    ['app17', 'eth1', 'static', '192.168.0.2', '255.255.255.0', '192.1
+68.0.1'],
);

my %hosts;

while (my @row = @{shift @data || []}) {
    push @{$hosts{$row[0]}}, \@row;
}

for my $host (keys %hosts) {
    genFile ($host, $hosts{$host});
}


sub genFile {
    my ($host, $rows) = @_;
    my $fileName = "/tmp/interfaces.$host";
    my @ifaces   = map {$_->[1]} @$rows;

    print "--------- $fileName --------------\n";
    print <<STR;
auto lo
iface lo inet loopback

auto @ifaces
STR

    for my $row (@$rows) {
        my ($hostname, $iface, $type, $address, $netmask, $gateway) = 
+@$row;

        if ($type eq "dhcp") {
            print "\niface $iface inet $type\n";
        } else {
            print <<STR;

iface $iface inet $type
address $address
netmask $netmask
network 192.168.0.0
broadcast 192.168.0.255
gateway $gateway
STR
        }
    }
}
[download]

Prints:

--------- /tmp/interfaces.app15 --------------
auto lo
iface lo inet loopback

auto eth0 eth1

iface eth0 inet dhcp

iface eth1 inet static
address 192.168.0.4
netmask 255.255.255.0
network 192.168.0.0
broadcast 192.168.0.255
gateway 192.168.0.1
--------- /tmp/interfaces.app12 --------------
auto lo
iface lo inet loopback

auto eth1

iface eth1 inet static
address 192.168.0.5
netmask 255.255.255.0
network 192.168.0.0
broadcast 192.168.0.255
gateway 192.168.0.1
--------- /tmp/interfaces.app17 --------------
auto lo
iface lo inet loopback

auto eth1

iface eth1 inet static
address 192.168.0.2
netmask 255.255.255.0
network 192.168.0.0
broadcast 192.168.0.255
gateway 192.168.0.1
[download]

The alternate approach uses the same output sub, but alters the way the data is managed up front. This code is slightly more complicated, but will work for huge databases where collating all the data in a hash is not practical - although it's hard to imagine when that would be an issue:

my @hostRows;

while ((my @row = @{shift @data || []}) || @hostRows) {
    if (@hostRows && (! @row || $row[0] ne $hostRows[0][0])) {
        genFile ($hostRows[0][0], \@hostRows);
        @hostRows = ();
        next if ! @row;
    }

    push @hostRows, \@row;
}
[download]

This code replaces the main line code following the data in the first version. The files generated are the same with the same content, although they will most likely be generated in a different order.