use warnings;
use strict;
$|=1;

my $cmd = q{
  perl -e 'print "child: id after exec: $$\n";
  print "child: sees processes...\n " ,
     grep { m/perl/ }
     `ps ax` , "\n";
  '
};

print "parent: pre-fork id $$\n";

if ( my $pid = fork ) {
  # this is parent code
  print "parent: post-fork id $$\n";
  print "parent: return from fork is: $pid\n";
  sleep 3;
  print "parent: still here, with id $$\n";
  print "parent: sees processes...\n " ,
     grep { m/perl/ }
     `ps ax` , "\n";
} else {
  # this is child code
  die "cannot fork: $!" unless defined $pid;
  print "child believes it is: $$\n";
  exec( $cmd ) # replaces child with a new perl: pid shouldn't change, right?
}