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

That can be more simply written as:

  $hash{$_}++;
[download]

But I think that the OP wants to increment the hash key instead of the value:

  $_++ while exists $hash{$_};
  $hash{$_}++;
[download]