$freq{substr $grade{$_}, 0, 1}++ for keys %grade;

    $freq{$_}++ for map { substr $_, 0, 1 } values %grade;

    my @letters = map { substr $_, 0, 1 } values %grade;
    $freq{$_}++ for @letters;

and

    open my $fh, '<:utf8', "grades.txt"
        or die "Failed to open file: $!";
    
    my %grade;
    while (<$fh>) {
        m/^(\w+) \s+ ([A-F][+-]?)$/x
        or die "Can't parse line '$_'";
        $grade{$1} = $2;
    };

    use 5.010;
    use IO::All -binary, -utf8;
    
    my $line_format = qr/
        ^               	    # line start
        (?<student>\w+)         # name
        \s+                     # gap
        (?<grade>[A-F][+-]?)    # grade
        $                       # line end
    /x;
    
    my $fh = io->file("grades.txt");
    my %grade;
    while (my $line = $io->getline) {
        die "Can't parse line '$line'" if $line !~ $line_format;
        $grade{$+{student}} = $+{grade};
    }