The naïve solution uses four nested loops, two (row+col) to find characters to match, then two (row+col) to find a pairing. That's rather expensive.

A cheaper solution would track the location of each character.

my @a = map { [ split // ] } qw(
   book
   reboot
   rocket
);

my %locs;
for my $i (0..$#a) {
   my $row = $a[$i];
   for my $j (0..$#$row) {
      push @{ $locs{$a[$i][$j]} }, "$i,$j";
   }
}

for my $c (sort keys %loc) {
   next if @{ $loc{$c} } <= 1;
   print("$c: ", join(' ', @{ $locs{$c} }), "\n");
}
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