As mentioned by fs, hashes would probably be the best way to go. Here is one way to do it, although I'm not trying to filter out any special characters:

use strict;

my %hash;
while (<DATA>) {
   chomp;
   my @chars = split '';
   for my $char (@chars) {
      # use the next line if case doesn't matter
      # $hash{lc $char}++;
      $hash{$char}++;
   }
   print join " ", @chars, "\n";
}

my $consensus = '';
for (sort keys %hash) {
   $consensus = $_ if $hash{$_} > $hash{$consensus};
   print "\$hash{$_}: $hash{$_}\n";
}
print "\nChar: '$consensus' has the highest count: $hash{$consensus}!\
+n";

__DATA__
aAabbbbcdDeeeeEEefFFffffffffffff
zzzzzzzxxxxxxxxxyyYyg
[download]

Output

a A a b b b b c d D e e e e E E e f F F f f f f f f f f f f f f 
z z z z z z z x x x x x x x x x y y Y y g 
$hash{A}: 1
$hash{D}: 1
$hash{E}: 2
$hash{F}: 2
$hash{Y}: 1
$hash{a}: 2
$hash{b}: 4
$hash{c}: 1
$hash{d}: 1
$hash{e}: 5
$hash{f}: 13
$hash{g}: 1
$hash{x}: 9
$hash{y}: 3
$hash{z}: 7

Char: 'f' has the highest count: 13!
[download]

--Jim