local $_ = $string;
my $total;
for my $letter (qw(A C G T)) {
    my $count =()= /$letter/g;
    print "$letter: $count\n";
    $total += $count;
}
my $other = length($_) - $total;
print "Other: $other\n";

##</code><code>##

local $_ = $string;
local %count;
s/([ACGT])/$count{$1}++;$1/ge;