# I've snipped out what I consider to be the important bits...
# Warning: This is untested code!

# First, store the sn count inside the DEVICE, since we have one DEVICE per $sn
struct ( DEVICE => { datetime => '$', devicetype => '$', zip => '$', version => '$', sn_count => '$'});

# Get the $sn and other fields here

# If this is the first time we see $sn, populate the DEVICE
if (!exists $SNs{$sn}) {
    $SNs{$sn} = new DEVICE;
    $SNs{$sn}->datetime('$datetime');
# etc. 
}

# update the sn_count for this $sn.
$SNs{$sn}->sn_count($SNs{$sn}->sn_count + 1);

foreach my $key (keys %SNs) {
    print "$key\n";
}