my %exerrors = map { 0+$_ => 1 } @exerrors;

foreach my $adv79key (keys %adv79) {
   if ( $exerrors{0+$adv79key} ) {
      print "$adv79key is an old error\n";
   } else { 
      print "$adv79key is a new error\n";
   }
}