I copied / pasted these lines into the prompts file. Then I opened a terminal. This was displayed by the terminal:
ubuntu@ubuntu-desktop:~$ perl /home/ubuntu/voxforge/HTK_scripts/prompts2mlf words.mlf prompts
writing to mlf file words.mlf
writing to words.mlf file done
A file with the name
words.mlf has been created in my home folder. It has this content.