$[

$[

This variable stores the index of the first element in an array, and of the first character in a substring. The default is 0, but you could theoretically set it to 1 to make Perl behave more like awk (or Fortran) when subscripting and when evaluating the index() and substr() functions.

As of release 5 of Perl, assignment to $[ is treated as a compiler directive, and cannot influence the behavior of any other file. (That's why you can only assign compile-time constants to it.) Its use is highly discouraged.

Prior to Perl v5.10.0, assignment to $[ could be seen from outer lexical scopes in the same file, unlike other compile-time directives (such as strict). Using local() on it would bind its value strictly to a lexical block. Now it is always lexically scoped.

As of Perl v5.16.0, it is implemented by the arybase module. See arybase for more details on its behaviour.

Under use v5.16 , or no feature "array_base" , $[ no longer has any effect, and always contains 0. Assigning 0 to it is permitted, but any other value will produce an error.

Mnemonic: [ begins subscripts.

Deprecated in Perl v5.12.0.