use strict;
my $nest;
for (<DATA>) {
    $nest++ if /\bloop/;
    $nest-- if /\bendloop/;
    $nest && /warn/ && print
}
__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;
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