Perhaps you can save some fiddly typing map'ing the product of two source arrays to produce the @letters array. It doesn't look quite as straightforward but might be less prone to typos.

use 5.026;
use warnings;

my @letters = do {
   my @chars   = ( q{a} .. q{z} );
   my @counts  = (
      9, 2, 2, 4, 12, 2, 4, 2, 9, 1, 1, 4, 2,
      6, 8, 2, 1, 6, 4, 6, 4, 2, 2, 1, 2, 1 );
   map { ( $chars[ $_ ] ) x $counts[ $_ ] } 0 .. 25;
   };

say join q{}, @letters;
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Produces:-

aaaaaaaaabbccddddeeeeeeeeeeeeffgggghhiiiiiiiiijkllllmmnnnnnnoooooooopp
+qrrrrrrssssttttttuuuuvvwwxyyz
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I hope this is of interest.