use 5.038;
use feature 'class';
no warnings 'experimental';

class WithACounter {
  my $next_count = 1; # class initialization, called once
  my sub last_one_was_bad() { 0 }
  my sub delete_last_one() { ...; }

  field $count = $next_count++;
  ADJUST {
    if (last_one_was_bad()) { delete_last_one(); $next_count--; }
  }
}