> for my $i (0 .. $#a) {

looks wrong: $a is an array ref there is no @a involved.

IIRC $#$a should do.

Note This code is untested. ;)

update

now tested! :)

  DB<109> $a=[1..10]                            # arrref
 => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

  DB<110> $#$a
 => 9

  DB<111> $a=[map [0..$_],0..3 ]                # AoA
 => [[0], [0, 1], [0, 1, 2], [0, 1, 2, 3]]

  DB<112> $#{$a->[2]}
 => 2

  DB<113> @b=(1..5)                             # @array
 => (1, 2, 3, 4, 5)

  DB<114> $#b
 => 4
[download]

though I couldn't find a good documentation yet! :(