I'm seeking for the _proper_ way to _signal_ a 404 to whatever visits the site (search engines, bots, etc.).
Right.... And you already shown that you know the proper way to signal a 301. You print 301. Now, do you still really not understand how to print 404? Let me explain how one can print 404.
print 404; # TADA! Magic!
Printing an optional message and mandatory network end-of-lines is left as an excercise for the reader.
Please come back to the site if you have problems printing a different number!