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foreach my $adv79key (keys %adv79) {
   my $old;
...
      print "$adv79key is a new error\n";
   }
}

foreach my $adv79key (keys %adv79) {
   if ( grep { $adv79key == $_ } @exerrors ) {
...
      print "$adv79key is a new error\n";
   }
}

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

...
      print "$adv79key is a new error\n";
   }
}