Perl: Make the decoder reentrant

This fix is for the equivalent bug in the encoder, except the encoder
already had the appropriate logic. Said logic was buggy in the encoder,
whereas this commit _adds_ the cloning logic to the decoder in the first
place.
Perl: En-/Decoder changelog and version bump
Perl: Better diagnostics for test failure
Perl: Add minor TODO note
Perl: Fix decoder problem related to needlessly re-cloning encoders

This is a bug introduced in the re-entrance fixes: I didn't
appropriately clear the "decoder is dirty" flag. :(
Perl: Fix "bytes_consumed" for cloned decoders