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}++
};