You're a long-standing monk here; is this the attitude that you like to find at the front gate every day? It probably took you more time to type up your example than it would to code the solution.

For the fixed-number-of-columns case, just nest loops.

my $struct = [ [], [], [] ];
for my $A (@{$struct->[0]})
{
  for my $B (@{$struct->[1]})
  {
    for my $C (@{$struct->[2]})
    {
       ...
    }
  }
}
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For the variable-number-of-columns case, a recursive solution or a stack iteration would work.