It makes no sense to use sysread on a file handle with the :utf8 layer because the read might have ended mid-character.

sysread-using code should look something like this:

binmode($fh);  # Or open with :raw

my $buf = '';
while (1) {  # Or this could be a select loop.
   my $rv = sysread($fh, $buf, BLOCK_SIZE, length($buf);
   die($!) if !defined($rv);
   last if !$rv;

   # Identify and extract message.
   # Using LF-terminated messages in this example.
   while (s/^([^\n]*)\n//) {
      my $msg = $1;
      process_message($msg);
   }
}

die("Premature EOF") if length($buf);
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In your case, you just want to blindly forward everything you receive, so simply avoid decoding or encoding!

binmode($fh_in);   # Or open with :raw
binmode($fh_out);  # Or open with :raw

while (1) {  # Or this could be a select loop.
   my $rv = sysread($fh, my $buf, BLOCK_SIZE, length($buf);
   die($!) if !defined($rv);
   last if !$rv;

   print($fh_out $buf);
}
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