idlwave-auto-fill-split-string

idlwave-auto-fill-split-string is a variable defined in `idlwave.el'.
Its value is

t

Documentation:
If non-nil then auto fill will split strings with the IDL `+' operator.
When the line end falls within a string, string concatenation with the
'+' operator will be used to distribute a long string over lines.
If nil and a string is split then a terminal beep and warning are issued.

This variable is ignored when `idlwave-fill-comment-line-only' is
non-nil, since in this case code is not auto-filled.

You can customize this variable.