It's UCS-2le. UTF-16le, a superset of UCS-2le, also works. (Case doesn't matter.) You should use UTF-16le when reading. You could use UCS-le for writing if you wanted to be strict, but UTF-16le should be fine too. (Just be aware that UTF-16 is a variable-width encoding, so there might be some differences of opinion as to how long a string is.)

Note that Perl might mangle some characters due to the unfortunate order in which the :crlf layer is placed in relation to the :encoding layer. You can avoid that problem using

# Preserve CRLF
open(my $fh, '<:raw:perlio:encoding(UTF-16le)', ...)

# CRLF->LF
open(my $fh, '<:raw:perlio:encoding(UTF-16le):crlf', ...)
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