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use strict;
use warnings;
...
pp $_ for @c;

pp \@c;

[[1, "a"], [2, "a"], [3, "a"]]
[[1, "b"], [2, "a"], [3, "a"]]
...
[[1, "b"], [2, "a"], [3, "b"]]
[[1, "a"], [2, "b"], [3, "b"]]
[[1, "b"], [2, "b"], [3, "b"]]

do {
  my $a = [
...
  $a->[7][1] = $a->[3][1];
  $a;
}

   for my $row (@old) {
      for my $r (@b) {

final:
[[1, "a"], [2, "a"], [3, "a"]]
...
[[1, "b"], [2, "a"], [3, "b"]]
[[1, "b"], [2, "b"], [3, "a"]]
[[1, "b"], [2, "b"], [3, "b"]]