The simplest way to do this in APL is by starting APL and typing this:

⎕←'Hello World!'

However, for the rest of the world this might not qualify as a program. Aplers are used to the idea of using the session as an input/output interface. Others might not be. In mswindows for instance a program is something we can call from the Run... dialog box.

To get our program into a state where we can do that requires a choice between a number of alternative routes probably none of which will actually use ⎕←.

Here are three possible approaches:

CategoryHelloWorld CategoryDyalog