$[      The index of the first element in an array, and of the first
               character in a substring.  Default is 0, but you could theoret‐
               ically set it to 1 to make Perl behave more like awk (or For‐
               tran) when subscripting and when evaluating the index() and
               substr() functions.  (Mnemonic: [ begins subscripts.)

               As of release 5 of Perl, assignment to $[ is treated as a com‐
               piler 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.

               Note that, unlike other compile-time directives (such as
               strict), assignment to $[ can be seen from outer lexical scopes
               in the same file.  However, you can use local() on it to
               strictly bind its value to a lexical block.

perl -wle'
$[ = 23;
@a = qw/ a b c /;
print $a[23]'
a