next unless length $deadstring; uses a statement modifier (the unless bit) which is rather like an if statement backwards. You can use if, unless, for and while as statement modifiers.

Try copying and pasting from your file into the sample code where the word list is. If that shows the error you see with the real file then post the modified sample here. If it doesn't show the same error then most likely your editor is modifying the text somewhat. One possibility is that the file has different line endings than are native on your system. For example, you may have got the file from a *nix system and are running it under Windows or vis-versa.