I appreciate that Laurent_R but the code cannot use buffered IO as it is using IO::Select which is why the code was changed to use sysread many years ago (apparently). sysread did/does support :utf8 although I realise it is unvalidated UTF-8.
The problem I am seeking an answer to is how to continue using sysread without getting the deprecated warnings OR how to change the code to use some other unbuffered read that supports UTF8 layer.
So far I have tried changing the layer on the read end of the socket to :unix (instead of UTF-8), reading the bytes and decoding them. This appears to work but has the caveate that if a part message was read the octets in it might fall in the middle of a UTF8 character.