my $pre_element;
my @stack;
while ($file =~ m{...regex for a pi...}) {
  my $pre = $1;
  ...
  if ($pi_cmd eq 'CLG.MDFO') {
    ...same as above...
  } else {
    if ($pre =~ m{...regex for element tag...\z}) {
      $pre_element = $1;
    }
    if ($pi_cmd = 'CLG.MDFC') {
      unless ($pre_element = pop(@stack)) {
        ...
      }
    }
  }
}