I was not commenting on the error being silent or not.

I am commenting that your initial version failed to log critical information that would make it possible to debug the problem later. And no matter how controlled the environment may be, you are not going to convince me that you won't ever have bugs to track down.

I am glad that your current example does that better. It does not do it well enough to make me happy, but it is better.

Here is a better version of the same thing that you have:

sub ret_read_fh {
  my $filename = shift;
  my $fh = do {local *FH};
  unless(open($fh, "< $filename")) {
    log_error("open '$filename': $!");
    return;
  }
  unless(flock($fh, 1)) {
    log_error("flock '$filename': $!");
    return;
  }
  return $fh;
}

sub display_values_from_file {
  my $fh = ret_read_fh(shift);
  if ($fh) {
    while(<$fh>) {
      print process_line_from_file($_);
    }
  }
  else {
    print "Error opening file.  Please try again later!";
  }
}
[download]

Why better? Well suppose that some bright sysadmin gets the idea of having your data mounted from another machine over NFS. (This is not an unreasonable thing for a sysadmin to have reason to do.) With the revised version you will get notified that flock no longer works. With the original version the error would be silent until someone noticed sporadic loss of data over 6 months and thought to wonder why. (At which point you read your code and have no a clue where it could fail.)

My points?

1. Don't trust system calls to succeed.
2. Make sure that when they go wrong you have enough information to debug.
3. Realistically you have a choice. Make errors fatal, or else encapsulate code which needs to handle errors. After all when you find a new possible cause of errors you do not want handling it to be harder than necessary.

Also a middle ground to consider is to make errors be fatal, but make sections with fatal error handling be trapped by an eval block. That makes breaking out on large numbers of possible errors easy to implement, while not interfering with your ability to handle the exception for the user.