use strict;
use warnings;
use feature 'say';
use autodie;
$, = ' ';

{ 
    open my $f, '>:raw:encoding(UTF-16LE):crlf', 'test';
    say $f 123;
}

{ # 1 "pure binary slurp"
    open my $f, '<:raw', 'test'; 
    undef local $/;
    say PerlIO::get_layers( $f );
    say unpack '(H2)*', <$f>;
}
{ # 2 OP's case
    open my $f, '<:encoding(UTF-16LE)', 'test';
    say PerlIO::get_layers( $f );
    say unpack '(H2)*', <$f>;
}
{ # 3 correct
    open my $f, '<:raw:encoding(UTF-16LE):crlf', 'test';
    say PerlIO::get_layers( $f );
    say unpack '(H2)*', <$f>;
}
{ # 4 correct
    open my $f, '<', 'test';
    binmode $f, ':raw:encoding(UTF-16LE):crlf';
    say PerlIO::get_layers( $f );
    say unpack '(H2)*', <$f>;
}
{ # 5 same as #2
    open my $f, '<', 'test';
    binmode $f, ':encoding(UTF-16LE)';
    say PerlIO::get_layers( $f );
    say unpack '(H2)*', <$f>;
}

__END__

unix crlf
31 00 32 00 33 00 0d 00 0a 00
unix crlf encoding(UTF-16LE) utf8
31 32 33 0d 0a
unix crlf encoding(UTF-16LE) utf8 crlf utf8
31 32 33 0a
unix crlf encoding(UTF-16LE) utf8 crlf utf8
31 32 33 0a
unix crlf encoding(UTF-16LE) utf8
31 32 33 0d 0a