Most things answered fellow Almut to you and to others practically the way i think, so thank you Almut. In one aspect i want still point:
(That actually happened once, with the introduction of Perl 5.8.0 on RedHat Linux: the particular RedHat release used utf-8 locale settings for the "default" shell environment, and that version of Perl used the locale settings in order to decide what the default i/o layer should be.
If those scripts were broke after making locale spread default, those script were buggy, right? No one should use locale pragma if they don't mean it, right?