I'm not sure if it will work in viemu, but in VIM you can do the following...
Using Yank and Paste
Yank the text to a specific register. Select the text in visual mode and use the command "ay to yank the text to the register a. Then when pasting call the command "ap, which pastes the contents of the a register.
Using Normal Command
But I would strongly prefer to use the normal command. Just select the lines
SOMETHING_XXXXXXXXXXXXXX_ELSE
SOMETHING_XXXXXXXXXXXXXX_ELSE2
SOMETHING_XXXXXXXXXXXXXX_ELSE3
SOMETHING_XXXXXXXXXXXXXX_ELSE4
using line visual mode (<C-v>) and then issue this command: :'<,'>normal fXct_HELLOWORLD. Then you'll have
SOMETHING_HELLOWORLD_ELSE
SOMETHING_HELLOWORLD_ELSE2
SOMETHING_HELLOWORLD_ELSE3
SOMETHING_HELLOWORLD_ELSE4
This means that it will run the command fXct_HELLOWORLD for each line. Let me explain the command:
- fX - moves the cursor until the first X;
- ct_ - deletes everything untill _ and puts you in insert mode;
- HELLOWORLD - the word which will substitute XXXXXXXXXXXXXX;