Using a char variable in tr///

tr/// doesn't work with variables unless you wrap it in an eval

But there is a nicer way to do this:

$count{$_} = () = $text =~ /$_/g for 'a' .. 'z';

For the TIMTOWTDI:

$count{$_}++ for grep /[a-z]/i, split //, $text;

tr doesn't support variable interpolation (neither in the search list nor in the replacement list). If you want to use variables, you must use eval():

$count{$c} = eval "\$text =~ tr/$c/$c/";

That said, a more efficient (and secure) approach would be to simply iterate over the characters in the string and increment counters for each character, e.g.:

my %count = map { $_ => 0 } 'a' .. 'z';

for my $char (split //, $text) {
    $count{$char}++ if defined $count{$char};
}

If you look at the perldoc for tr/SEARCHLIST/REPLACEMENTLIST/cdsr, then you'll see, right at the bottom of the section, the following:

Because the transliteration table is built at compile time, neither the SEARCHLIST nor the REPLACEMENTLIST are subjected to double quote interpolation. That means that if you want to use variables, you must use an eval():

eval "tr/$oldlist/$newlist/";
die $@ if $@;
eval "tr/$oldlist/$newlist/, 1" or die $@;

Thus, you would need an eval to generate a new SEARCHLIST.

This is going to be very inefficient... the code might feel neat, but you're processing the complete string 26 times. You're also not counting uppercase characters.

You'd be better off stepping through the string once and just incrementing counters for each character found.