I'd use the List::Util module to randomize the input, then drop the randomized letters in place.

use List::Util qw(shuffle);

my @letters = shuffle(A .. Z);

print @letters;
# prints something like WOGJLMXQCDVIPNESURZHTFYKBA

for $r (1 .. 2) {        # or whatever size you need
    for $c (1 .. 13) {   # same here
        $data[$r][$c] = shift @letters;
    }
}
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