# Case 1: don't count unwanted characters
for my $char(@letters) {
    ++$charCount{$char} if $char !~ /[a-zA-Z]/;
}

# Case 2: delete unwanted characters
my %t = %charCount;
$t{$_}=$charCount{$_} for grep {/[a-zA-Z]/} keys %charCount;
%charCount=%t;

# Case 3: ignore unwanted items during report
for my $char (sort keys %charCount) {
    next unless $char =~ /[a-zA-Z]/;
    # print report entry
}