The Flag FB_QUIET seems to be the answer
providing a call back helps identifying the malformed bytes

Unfortunately, that doesn't always seem to be the case:

use warnings;
use strict;
use Encode qw/decode/;
use Data::Dump;

dd decode('UTF-16-BE', "\xD8\x3D\xDD\xFA", Encode::FB_QUIET|Encode::LE
+AVE_SRC);
dd decode('UTF-16-BE', "\x3D\xDD\xFA", Encode::FB_QUIET|Encode::LEAVE_
+SRC);
dd decode('UTF-16-BE', "\x3D\xDD\xFA", sub{ sprintf "<U+%04X>", shift 
+});

dd decode('UTF-16-LE', "\x3D\xD8\xFA\xDD", Encode::FB_QUIET|Encode::LE
+AVE_SRC);
dd decode('UTF-16-LE', "\xD8\xFA\xDD", Encode::FB_QUIET|Encode::LEAVE_
+SRC);
dd decode('UTF-16-LE', "\xD8\xFA\xDD", sub{ sprintf "<U+%04X>", shift 
+});

__END__

"\x{1F5FA}"
"\x{3DDD}"
"\x{3DDD}"
"\x{1F5FA}"
"\x{FAD8}"
"\x{FAD8}"

[download]

It could be argued that this is a bug / oversight in Encode, and of course if we know we're reading UTF-16 we should always read an even number of bytes. But still, because of this behavior, and because I don't yet know if there are encodings where chopped up byte sequences might end up in valid characters, I'm doubtful that a generalized "read any file backwards with Encode" is reliable. Personally I'd just make a version for UTF-8 and UTF-16, and any others as needed, or other encodings can be converted to the supported ones.

In reply to Re^5: Processing an encoded file backwards by haukex
in thread Processing an encoded file backwards by LanX

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