use Unicode::UCD qw(charinfo);
my %category_map = (
    Lu => 'U', # letter, upper
    Ll => 'L', # letter, lower
);

my $r = 'SomEThingWithMIXEDRegiSter';
$r =~ s
    {
        (.) # capture the character
    }
    {
        $category_map{
            charinfo(ord $1)->{category}
        }
    }egmsx;
# ULLUULLLLULLLUUUUUULLLULLL