The most likely reason -- especially given the network involvement -- is TOCTTOU.

I don't see the possibility for a race condition here. A race condition could happen, if something changes to the file between the time I check the existence, and the time I read the file. However, the file is pretty static: It is kind of a configuration file, and after it is created, it doesn't change for weeks, maybe months.
-- 
Ronald Fischer <ynnor@mm.st>

A race condition could happen, if something changes to the file between the time I check the existence, and the time I read the file. However, the file is pretty static:

With the network involved, it doesn't have to be the file that changed. Any temporary resource state in the fabric between you and the actual file could -- breifly -- manifest itself as in an inability to open the file.

I'd suggest cutting out the possibility of a race by just opening the file. And if the open fails, retry some number of times before logging an error:

for my $try ( 1 .. RETRIES ) {
  if(open(my $rule_fh,'<',$rule_file)) {
    last;
  }
  if( $try == RETRIES ) {
    die "Can not read file $rule_file : $! / $^E";
  }
}
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