open IN, '<:raw:encoding(UTF-16LE)', $ARGV[ 0 ];
open OUT, '>:raw:encoding(UTF-16LE)', $ARGV[ 0 ] . '.modified';
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For text files, I'd suggest using :raw:encoding(UTF-16LE):crlf:utf8 instead, in which case Perl will do proper linefeed translations (on Windows), just like it would without the :encoding(UTF-16LE).  Otherwise, be prepared to handle trailing carriage returns (\r) yourself — chomp (with the default $/ setting), for example, will not remove them...