use Fcntl qw (:DEFAULT :flock :seek);
my $file = '/dev/shm/tmpcount.txt';

for (1 .. 180) {
  print "$_\n";
  &filewrite($file,$_);
  sleep 1;
}

sub filewrite {							
  my ($file,$enc) = @_;
  open(my $fh,">", $file) || die "Can't open $file for reading: $!";
  my  $mode = LOCK_EX; 
  flock($fh,$mode) or die "Couldn't lock $file for write";
  print $fh $enc;
  close $fh;
}