my @if_stack;
while (<$input>)
{
  if (/$start/) {
    push @if_stack, $_;
    next;
  }
  if (/$stop/) {
    die unless @if_stack;
    pop @if_stack;
    next;
  }
  if (@if_stack) {
    # got a line which is inside a conditional
  }
  else {
    # got a line which is not inside a conditional
    print $output $_;
  }
}