arybase

           $[ = 1;

           @a = qw(Sun Mon Tue Wed Thu Fri Sat);
           print $a[3], "\n";  # prints Tue

DESCRIPTION
       This module implements Perl's $[ variable.  You should not use it
       directly.

       Assigning to $[ has the compile-time effect of making the assigned
       value, converted to an integer, the index of the first element in an
       array and the first character in a substring, within the enclosing
       lexical scope.

       It can be written with or without "local":

           $[ = 1;
           local $[ = 1;

       It only works if the assignment can be detected at compile time and the
       value assigned is constant.

       It affects the following operations:

           $array[$element]
           @array[@slice]
           $#array
           (list())[$slice]
           splice @array, $index, ...
           each @array
           keys @array

           index $string, $substring  # return value is affected
           pos $string
           substr $string, $offset, ...

       As with the default base of 0, negative bases count from the end of the
       array or string, starting with -1.  If $[ is a positive integer,
       indices from "$[-1" to 0 also count from the end.  If $[ is negative
       (why would you do that, though?), indices from $[ to 0 count from the
       beginning of the string, but indices below $[ count from the end of the
       string as though the base were 0.

       Prior to Perl 5.16, indices from 0 to "$[-1" inclusive, for positive
       values of $[, behaved differently for different operations; negative
       indices equal to or greater than a negative $[ likewise behaved
       inconsistently.

HISTORY
       Before Perl 5, $[ was a global variable that affected all array indices
       and string offsets.

       Starting with Perl 5, it became a file-scoped compile-time directive,
       which could be made lexically-scoped with "local".  "File-scoped" means
       the Perl core.  The erratic behaviour that occurred with indices
       between -1 and $[ was made consistent between operations, and, for
       negative bases, indices from $[ to -1 inclusive were made consistent
       between operations.

BUGS
       Error messages that mention array indices use the 0-based index.

       "keys $arrayref" and "each $arrayref" do not respect the current value
       of $[.

SEE ALSO
       "$[" in perlvar, Array::Base and String::Base.

perl v5.22.1                      2018-11-19                    arybase(3perl)
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