I came up with this. It's less code than yours (well, if you spread your code out a bit :), but it's actually a bit slower, in benchmarking. So there may be better ways of going about it.

#!/usr/local/bin/perl -w
use strict;

my %count;

s/([a-zA-Z])/{ $count{lc $1}++; $1 }/eg
    while <>;

for my $letter (sort keys %count) {
    print $letter, "=", $count{$letter}, "\n";
}
[download]

It works by finding a letter ("a-zA-Z"), lower-casing it, and increasing the count of that letter; and it does so for each letter that it finds.