Mi solution to Ovid puzzle...

gistfile1.pl

use strict;
use warnings;

my @a = ( [ 'b', 'c', 'f' ],
          [ 'a', 'd' ],
          [ 'c', 'd', 'e' ] );

my $last_ix = -1;
my %ixs;

my @b = map {
    my @out;
    $out[$ixs{$_} //= ++$last_ix] = $_ for @$_;
    \@out;
} @a;

$#$_ = $last_ix for @b;

use Data::Dumper;
print Dumper \@b;