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How do I find out if I'm running interactively or not?
Good question. Sometimes -t STDIN and -t STDOUT can give clues, someti
+mes not.

...
    } else {
        print "background\n";
    }

    $on_a_tty = -t STDIN && -t STDOUT;
    sub prompt { print "yes? " if $on_a_tty }
    for ( prompt(); <STDIN>; prompt() ) {
        # do something
    }