LABEL O 1
  blah
STOP

LABEL C A
  blah
STOP

LABEL O 3
  blah
STOP


##</code><code>##

while (<IN>) {
    my $exp_idstr = sprintf( "%o", $expected_id );
    if ( s/\(LABEL O $exp_idstr\)\n/$1  NEWSTUFF/ ) {
...