No need to sort

sub next_key {
   my ($h, $key) = @_;

   my $best;
   foreach my $hkey (keys %$h) {
      if ( $hkey >= $key ) {
         if ( !defined($best) || $hkey < $best ) {
            $best = $hkey;
         }
      }
   }

   return $best;
}
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