It indeed appears to be a bug in PerlIO::encoding. See http://cpansearch.perl.org/src/RJBS/perl-5.18.1/ext/PerlIO-encoding/encoding.xs to note that "encode" and "decode" methods are simply called with a buffer full of bytes with no attempts to handle incomplete multi-byte characters across buffer boundaries.

To fix this efficiently, you'd want Encode's encode() and decode() methods (or similar) to support "tell me how many bytes on the end to save for later as they could be incomplete multi-byte characters in the desired encoding". Ah, I see FB_QUIET is already there for just that purpose. Unfortunately, using that completely defeats the purpose of allowing options like FB_WARN and FB_CROAK. Plus I don't see how that code makes it reasonable to detect invalid characters instead of just ending up in an endless loop of converting 0 bytes over and over.

It would be helpful for something similar to FB_QUIET to be defined as a bit that can be combined with FB_WARN or FB_CROAK such that failing at the first byte or (better) too far before the end of the buffer triggers the warn/croak but failing with a single, incomplete fragment of a multi-byte character on the end of the buffer acts like FB_QUIET would.

But surely that's already plenty of information for you to file the bug report, eh?

The error is already demonstrated in the OP. The error is not in the output, but in the fact that in the second case an error (warning) message is emitted when there is no error to report. The fact that there is no error to report is demonstrated by the fact that the output of the two cases is identical (nearly, the second output has an additional character at the beginning) and neither are garbled or erroneous in anyway.

--DrWhy

"If God had meant for us to think for ourselves he would have given us brains. Oh, wait..."