sub _index_of_largest
{
    my $X = shift;  my $L = shift;
    my $x = $#_;    my $l = pop;
    defined $X and $L > $l and ($x,$l) = ($X,$L);
    @_ or return $x;
    unshift @_, $x, $l;
    goto &_index_of_largest
}

sub index_of_largest
{
    @_ or die;
    _index_of_largest(undef,undef,@_);
}

my $i = index_of_largest( @a );