my $sock_in       = ...;
my $fh_out        = ...;
my $bytes_to_read = ...;

while ($bytes_to_read > 0) {
   my $bytes_read = read($sock_in, my $block = '', $bytes_to_read);

   if (!$bytes_read) {
      my $msg = defined($bytes_read) ? 'Socket closed unexpectedly' : $!;
      die("Unable to read from sender: $msg\n");
   }

   $bytes_to_read -= $bytes_read;   
   print $fh_out $block;
}