if(exists $hash{$_}){ # exists, so increment
    $hash{$_}++;
  }
  else{                 # doesn't exist, so set to 1
    $hash{$_} = 1;
  }

##</code><code>##

  $hash{$_}++;

##</code><code>##

  $_++ while exists $hash{$_};
  $hash{$_}++;