sub checkLine {
    # returns an (error) string that describes
    # what the line contained.
    # I place the if behind the return because
    # if I look at the code, I have an error message
    # and want to know what caused this message.
    return "! found at start of line." if /^!/;
    return "Line too short" if !/.{5,}/;
    return "";
  };

if {/BEGIN/i) {
  # here I use a prefixed "if", because a longer block follows.
};