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 bound its value to a lexical block.
       from perldoc perlvar

So I don't have to worry that someone will change mine $[ from within a different file that I used, but I still might run into problems if someone fiddles with $[ somewhere above my code in the same file.

@a = (0,1,2,3,4,5);

sub pr {
    print "sub \$a[2]=$a[2]\n";
}
print "\$a[2]=$a[2]\n";
pr;

{
  $[ = 1;
  print "\$a[2]=$a[2]\n";
  pr;
}

print "\$a[2]=$a[2]\n";
pr;
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I refuse to worry anyway.