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

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

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