Don't have the cycles to test, but try the PrimType in case the 5.5 API parser is sensitive to the soap XSD type:

my $io_alloc = StorageIOAllocationInfo->new (shares => $sharesInfo, limit=> PrimType->new(10, 'long'));

$device->storageIOAllocation ($io_alloc);

my $virtualDeviceConfigSpec = VirtualDeviceConfigSpec->new (device => $device, operation => VirtualDeviceConfigSpecOperation->new('edit'));

my $vmConfig = VirtualMachineConfigSpec->new (deviceChange => [$virtualDeviceConfigSpec]);

$vm->ReconfigVM (spec => $vmConfig);

If that doesn't work, I'll see if I can reproduce it in my lab environment, but always possible it's a bug.