$syb = 'aaaatttt';
$syb =~ s/
(?:(?<!a)a|(?<!t|a)t)
(?:(?<=a)a|(?<=a)t|(?<=t)t){3,6}
(?:(?<=a)(?!a)|(?<=t)(?!t))
/\U$&/gix;
print $syb;

AAAAtttt