Copy delta file of virtual disk (snapshot file) using vsphere sdk for perl

[Afgdd]

I can use method CopyVirtualDisk_Task (https://www.vmware.com/support/developer/converter-sdk/conv60_apireference/vim.VirtualDiskManager.ht...) to copy normal virtual disk file (.vmdk). But when I copy a delta file (for example -000001.vmdk), the delta file will be change to a normal disk file (it does not keep format). I added parameter destSpec for method CopyVirtualDisk_Task to specify virtualDiskType is delta, but I got error: "The requested operation is not implemented by the server". I'm using vsphere sdk for Perl 5.5, and follow the link https://www.vmware.com/support/developer/converter-sdk/conv60_apireference/vim.VirtualDiskManager.Vi..., delta virtualDiskType is supported from API v5.5. Does anybody know how to resolve this problem?