This already prints
Hello, World!, but the newline is missing. The input is just placed in
\x which is never used. In order to be able to modify the output, I guess we need to do some preparations (like, "lay a trap") before calling this code, so
\x is actually expanded. It can be done when invoked like this (assuming the original source is in
x.tex in the current directory):
This first redefines
\vfill (which is part of the expansion of
\bye) to expand to our
\x, so now we can provide some input printing the missing newline:
This sets the character used for newlines in
@ and immediately uses it.
This whole thing modifies the environment the original program will run in, so if there are any objections to this method, I'll retract it.