I thought I might be able to pass a negative number to FETCH if the index was negative and bigger than the number of elements.

Turns out FETCH is not even called in that situation, even though FETCH *is* called for indexes greater than the highest existing index.

use Tie::Array qw( );
our @ISA = 'Tie::StdArray';

sub FETCH {
   my ($self, $idx) = @_;
   warn("Fetching index $idx\n");
   return $self->SUPER::FETCH($idx);
}

my @a;
tie @a, __PACKAGE__;
$#a = 4;

my $s;
$s = $a[-@a+1];  # Fetching index 1
$s = $a[-@a];    # Fetching index 0
$s = $a[-@a-1];  # FETCH not called!

$s = $a[@a];     # Fetching index 5
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Perl 5.6.1 & 5.8.6