Set to a non-zero integer value to do multi-line matching within a string, 0 (or undefined) to tell Perl that it can assume that strings contain a single line, for the purpose of optimizing pattern matches. Pattern matches on strings containing multiple newlines can produce confusing results when $* is 0 or undefined. Default is undefined. (Mnemonic: * matches multiple things.) This variable influences the interpretation of only ^ and $. A literal newline can be searched for even when $* == 0.
Use of $* is deprecated in modern Perl, supplanted by the /s and /m modifiers on pattern matching.
Assigning a non-numerical value to $* triggers a warning (and makes $* act if $* == 0), while assigning a numerical value to $* makes that an implicit int is applied on the value.