for (sort keys %histogram) {
  ...
};

##</code><code>##

my %validchars = map { $_ => uc $_ } (qw( A B C D E F ... Z a b c d ... z ));
# this sets
#   $validchars{A} = A
#   ...
#   $validchars{Z} = Z
#   $validchars{a} = A
#   ...

# and the later:

if (exists $validchars{$_}) {
  my $target = $validchars{$_}; 
  $histogram{$target}++
};

##</code><code>##

my %validchars;
if ( $arg eq '-a' ) {
  %validchars = map { $_ => uc $_ } ( \x00 .. \xff );
} elsif ( $arg eq '-c' ) {
  %validchars = map { $_ => $_ } ( 'A' .. 'Z', 'a'..'z' );
};