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);

##</code><code>##

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);
}