I wasn't expecting this behaviour. I mistakenly thought that adding the :raw layer to the socket would set it up to accept any arbitrary bytes without worrying about what they were

The problem is that if the scalar has its utf8 flag set then it is not a string of bytes but a string of characters. The bytes required to represent those characters will depend on which encoding you want to use - hence the need to specify an encoding either with binmode or by explicitly calling encode.

Turning off the utf8 flag as you suggest is not a good solution since it will cause the characters to be sent as the bytes used by Perl's internal string representation. This internal representation is conceivably subject to change (probably unlikely), but more importantly it's not a standard format (it's almost but not exactly UTF8).