#!/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;
}