I have to say that according to the output of the following test script, the value of $[ is definitely not read-only at run-time. You can keep assigning a new number to $[ and change the subscript behaviour at run-time. However the use of $[ = $a is not supported. The Perl documentation clearly states that $[ is treated as a compiler directive and its use is discouraged. I suspect that Perl handles this special variable differently because of efficiency reasons (or because the whole idea of $[ was a hack in the first place afterall).
my @elements = qw/ 0 1 2 3 4 5 6 7 8 9 /;

$[ = 0;
print $elements[1], "\n";

$[ = 1;
print $elements[1], "\n";

if ($[ == 1) {
  $[ = 0;
  print $elements[1], "\n";
}

# The following will fail at compile time
#   $a = 0;
#   $[ = $a;  # reset
#   print $elements[1], "\n";

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And the output is -
1
0
1

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In reply to Re: *a=*[; bug? by Roger
in thread *a=*[; bug? by ambrus

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