I am unable to duplicate what you describe. I modified your code to see if the pid file contains a pid after returning from the wait() call, and it does: 

#!/usr/bin/perl

use strict;
use warnings;

my $fork_pid = fork();

if ($fork_pid == 0) {
  write_pid();
  exit 0;
}
elsif ($fork_pid) {
  wait;
  open my $FILEH, '<', 'mypid.txt';
  my $line = <$FILEH>;
  close $FILEH;
  print "I am $$. Saw child pid $line";
}
else {
  die "Couldn't fork";
}

sub write_pid {
  open my $FILEH, '>', 'mypid.txt';
  print $FILEH $$ . "\n";
  close $FILEH;
}

[download]

When I run it, I get this: 

$ ./blah.pl
I am 7202. Saw child pid 7203

[download]

Indicating that the pid file is being written properly with the child's pid.

In reply to Re: Fork child process always finishes with parent by markkawika
in thread Fork child process always finishes with parent by Rogue

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