There are two problems here:
1) count the occurance of words
2) display the words and occurances in decending order.

Try this...

my %count;
for (@letters) {
  $count{$_}++;
}

for ( map { $_->[0] }
      sort { $b->[1] <=> $a->[1] }
      map { [ $_, $count{$_} ] } keys %count )
{
  print $_, "=>", $count{$_}, "\n";
}
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