UCS-2 is a strict subset of UTF-16, with no BOM and just the BMP (“Basic Multilingual Plane”).
UTF-16 is handled internally and rather fast by Expat (the core element of XML::Parser), much faster than the decodings done through *.enc files.
Since around 2006 and despite supposedly un-enumerable requests for ucs-2.enc requestors always got told to use UTF-16 instead of UCS-2.
BTW: the long name for UCS-2 is ISO-10646-UCS-2.
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