idlwave-split-line-string

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

t

Documentation:
If non-nil then `idlwave-split-line' will split strings with `+'.
When the splitting point of a line falls inside a string, split the string
using the `+' string concatenation operator. If nil and a string is
split then a terminal beep and warning are issued.

You can customize this variable.