$SIG{CHLD}='IGNORE';

#...

unless(fork())
{
  SIG{CHILD}='DEFAULT';
  close(STDIN);
  close(STOUT);
  close(STERR);

  open(my $mailfh, '| /usr/sbin/sendmail -t -oi ') or exit();
  print $mailfh 'From: test@example.org', "\n",
        'To: admin@example.org', "\n",
        'Subject: ERROR in blah blah', "\n",
        "\n",
        'Error occured in ThisOrThat', "\n",
        "" . localtime(time), "\n",
        '.';
  close($mailfh);

  exit();
}