use strict;
if (defined(my $pid=fork()) {
  if ($pid) {
  # $pid is not 0, so this is the parent
  # Let's redirect the user (more extensive code in your example).
  redirect_user();
  } else {
  # $pid=0, so this is the child
  my $retval=do_child_code();
  exit $retval;
  }
} else {
# Oops, fork() returned undef - something is definitely wrong.
  DieNice("Unable to fork: $!\n");
}

sub do_child_code {

  # Do your time consuming stuff, setting $retval to
  # a useful number
  return $retval;
}