use strict;
use Fcntl qw(:DEFAULT :flock);
use FindBin qw($Bin);
use lib "$Bin";
use JSON;
use POSIX;



my $JSON_FILE = $ARGV[0];

open my $file, '<', $JSON_FILE or die("$JSON_FILE Could not open file:
+ $!\n");

my $data;
eval {
  $data = decode_json(<$file>);
  print $data;
};
if ( $@ ){
  print "Json file parsing failed.\n";
} 

my $targets = $data;

my $filename = "/tmp"; 
my $newfilename = "/filename";
open (my $fh, ">", $filename) or die "Could not open file '$filename' 
+$!\n";
print $fh "message..\n";
print $fh "message...\n";

my $time = strftime('%d-%m-%Y %H:%M:%S',localtime);

my $logfile = "/var/log/mylog.log";
open (my $logfile, ">>", $logfile) or die "Could not open file '$logfi
+le' $!";

for (my $index=0; $index <= $#$targets; $index++){    
  my $array = $targets->[$index]{labels};
  my $name = $array->{name};
  
  $time = strftime('%d-%m-%Y %H:%M:%S',localtime);
  
  # Set a timeout period
  $SIG{ALRM} = sub { flock $logfile, LOCK_EX;
                     print $logfile "message...\n"; 
                     flock $logfile, LOCK_UN;
                     flock $fh, LOCK_EX;
                     print $fh "message...\n";
                     flock $fh, LOCK_UN;
  alarm(60);
  
  defined(my $pid = fork) or die "fork failed: $!";
  unless( defined($pid) ) {
    flock $logfile, LOCK_EX;
    print $logfile "message...\n";
    flock $logfile, LOCK_UN;
  }

  unless($pid) { # child
    print "child: $$\n";
    my $var;
    eval {
      do something here...
    };
    if( $@ ) {
      flock $logfile, LOCK_EX;
      print $logfile "message...\n"; 
      flock $logfile, LOCK_UN;
    }
    unless( $var ) {
      flock $logfile, LOCK_EX;
      print $logfile "message...\n";
      flock $logfile, LOCK_UN;
      flock $fh, LOCK_EX;
      print $fh "message...\n";
      flock $fh, LOCK_UN;
    }
    else {
      flock $fh, LOCK_EX;
      print $fh "message...\n";
      flock $fh, LOCK_UN;
      flock $logfile, LOCK_EX;
      print $logfile "message...\n";  
      flock $logfile, LOCK_UN;
    } 
    close($fh);
    close($logfile);
    exit; # exit child process
  } 
  waitpid($pid,0); # could I refine like this?
  print "Parent Process";
  close($fh); # Close file handler $fh
  close($logfile); # Close file handler $logfile    
  rename ($filename, $newfilename) or die "Error in renaming $!";
  alarm(0); 
}
[download]

WonderG your usage of waitpid looks ok to me. The rename error: your $filename is /tmp which in a Unix-based system is usually a directory. So perhaps, you meant it as a basename and $filename changes in every run, in which case investigate using die "Error in renaming '$filename' to '$newfilename', $!" and check that you have read permissions for the source (and it does exist) and write permissions in the directory of the destination.

Thank you very much. I refined my code like below. I put waitpid in the "for" loop, but move print and rename out of the "for" loop, then everything is ok. Is it the right behavior?

    close($fh);
    close($logfile);
    exit; # exit child process
  } 
  waitpid($pid,0); 
}

print "Parent Process";
close($fh); # Close file handler $fh
close($logfile); # Close file handler $logfile    
rename ($filename, $newfilename) or die "Error in renaming $!"; # Rena
+me the temporary to avoid node exporter seeing half a file.
alarm(0);
[download]