use strict;
use warnings;

my $count;
while (my $line = <DATA>) {
	if ($line =~ m/endloop/) {
		$count --;
	} elsif ($line =~ m/loop/) {#don't put this before endloop, as endloop itself matches loop
		$count ++;
	} elsif ($line =~ m/warn/) {
		print $line if ($count > 0);
	}
}

__DATA__
begin
  loop
    warn('xxx');
  endloop;

  loop
    null;
      loop
        warn('AAA');
      endloop;
  endloop;
  
  loop
      loop
        warn('BBB');
      endloop;
    warn('rrr');
  endloop;

 begin
   loop
   endloop;

    warn('yyy');
 end;

  warn('zzz');
end;