Try this. Path::Tiny::spew writes to a temporary file and then atomic renames to the final name. This should never allow an empty file.

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11164885
use warnings;
use Path::Tiny;

#one.pl

my $file = '/dev/shm/tmpcount.txt';

for (1 .. 180) {
  print "$_\n";
#  &filewrite($file,$_);
  path($file)->spew($_); # NOTE writes to temp file, then atomic renam
+es over
  sleep 1;
}

#two.pl

my $file = '/dev/shm/tmpcount.txt';
my $c;

do {
  if (-e $file) {
#    $c = &fileread($file);
    $c = path($file)->slurp;
    die "C is empty" unless ($c);
    print "count is $c \n";
  } else { print "File not found\n"; }
} until ($c == 180);
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