$idx   = $#{$a1};       # index of last element
$idx   = $#$a1;         # same

$count = @{$a1};        # number of elements
$count = @$a1;          # same

$item  = $a1->[1];      # element 1
$item  = ${$a1}[1];     # same

@slice = @{$a1}[0, 1];  # slice of elements