AT
Perl deserves a mention here in any discussion of side-effects. It has a magic argument, $_, which means something like, “the return value of the previous call.” It gets used and/or changed by many of the core library functions, implicitly. As far as I know this gives Perl the distinction of being the only language where one global side-effect is considered a core feature.
http://blog.jenkster.com/2015/12/which-programming-languages-are-functional.html