Althought, I don't have an immediate direct answer to your question, I felt that in the mean time I could suggest a few improvements to your subroutine. I'll update this node if I find a suitable explanation for your problem at hand, though.

Normally I feel it's a good idea to separate the code that reports something from the code that simply does something. Therefore, as an improvement to your sub, I would move the call the fatal_error() method (which simply prints an error string to the client) outside of your subroutine and let the caller script handle the outcome of the safe_open() method. You could also look into making your method die with a custom error message, which could be picked up by a caller script using the try { }; if ($@) { ... } construct. So, the sub would look similar to this: 
# MAIN

my $fn = shift;

try {
  safe_open($fn);
};

if ($@) {
  # here, print fatal error
  fatal_error($@);
}

# SUBS
sub safe_open {
    my $fn = shift;
    my $fh = do { local *FH };
    my ($mode, $result);

    # reset the do_trun variable.
    # and also, make it local to this sub.. why keep it global? ;)
    my $do_trun;

    if ($fn =~ /^>[^>]/) {
        $mode = 1;
    }
    elsif ($fn =~ /^>>/) {
        $mode = 2;
    }

    if ($mode == 1 and $flock_enabled) {
        # Write mode with flock.
        $do_trun = 1;
        sysopen($fh, $fn, O_CREAT | O_WRONLY)
           or die "Failed to open '$fn' in write mode with flock.";
    }
    elsif ($mode == 1) {
        # Write mode without flock.
        open($fh, ">$fn")
          or die "Failed to open '$fn' for writing (no flock).";
    }
    elsif ($mode == 2) {
        # Append mode.
        open($fh, ">>$fn")   
         or die "Failed to open '$fn' for appending.";
    }
    elsif (!$mode) {
        open($fh, $fn);
          or die "Failed to open '$fn'."; 
    }

    if ($flock_enabled) {
      flock($fh, 2) or die "Failed to flock '$fn'.";
    }

    if ($do_trun) {
      truncate($fh, 0) or die "Could not truncate file $fn.";
    }

    return $fh;
}

[download]


Update: Err.. I guess this is a classical example of how things might go wrong when you resort to using global variables. From the code, I can't quite see where you receive/get/set/modify the $do_trun variable. It might be that you still have it set to some positive (true) value on consequent calls to the method, which causes your file to be truncated. I guess, you'll simply have to reset the $do_trun variable to 0 every time inside the actual sub to eliminate the 'once in a blue moon' phenomena. ;-)

_____________________ 
# Under Construction

[download]

In reply to Re: Safe way to open files by vladb
in thread Safe way to open files by L0rdPhi1

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