$string =~ s/(([A-Z])\2+)/lc $1/eg;

##</code><code>##

$string =~ s/([A-Z]{2,})/lc $1/eg;

##</code><code>##

$string =~ s/(([:upper:])\2+)/lc $1/eg;
$string =~ s/([:upper:]{2,})/lc $1/eg;