Update: danger++ made some excellent improvements.

As has already been hinted, at put a my $do_trun; at the top of the routine. use strict and warnings to avoid such problems in the future.

Also, I want to point out something else: if ($fn =~ /^>[^>]/) obviously expects the string to already have the angle brackets embedded. But then you go and add some more in $result = open($fh, ">$fn"); and $result = open($fh, ">>$fn");

And while not detrimental, they shouldn't be in sysopen($fh, $fn, O_CREAT | O_WRONLY); either. Together with the also already suggested removal of your fatal_error calls, I suggest the following: 
sub safe_open {
    my $fn = shift;
    # you need not initialize $fh anymore in Perl >= 5.6
    my ($fh, $mode, $result, $do_trun);

    $fn =~ s/^(>>?)//;
    $mode = length($1);

    if ($mode == 1) {
        # write mode
        if($flock_enabled) {
            $do_trun = 1;
            $result = sysopen($fh, $fn, O_CREAT | O_WRONLY);    
        }
        else {
            $result = open($fh, ">$fn");
        }
    }
    elsif ($mode == 2) {
        # append mode
        $result = open($fh, ">>$fn");
    }
    else {
        # read mode
        $result = open($fh, $fn);
    }

    $result or die "Couldn't open $fn: $!";
    flock($fh, 2) or die "Couldn't flock $fn: $!" if $flock_enabled;
    truncate($fh, 0) or die "Could not truncate file $fn: $!" if $do_t
+run;

    return $fh;
}

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

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