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#!/usr/bin/perl

...
my @c = map { my $t = $_; [ map [ $_, $t->[$_-1] ], 1..@a ] } @bc;

pp @c;

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