use Encode;
use PerlIO::encoding;
$PerlIO::encoding::fallback = Encode::FB_DEFAULT;
                              # BTW, why is FB_DEFAULT not the default?

my $malformed = "a\x00b\x00c\x00\xF4\xDBd\x00e\x00f\x00\n\x00"
              . "g\x00h\x00i\x00\n\x00";

                # = "abc<some junk>def\nghi\n" in UTF-16LE

open my $fh, "<:encoding(UTF-16LE)", \$malformed or die $!;
while ( my $u = <$fh> ) {
    print $u;
}
close $fh;