There essentially are no details.

Perl watches for key presses. Some other program generates a plot.

When Perl sees that an arrow key has been pressed it sends that information to the other program or programs. And they respond to it. Since everything is being done at least 10 times a second it appears that the key press responses are instantaneous. And in the end it is just easier to have a single Perl program controlling the others. That way multiple languages can be used and there aren’t any key press information conflicts.

For development purposes, the other programs can have internal routines that enable them to either respond to the key press information from Perl or from their own key read statements.