$s = 'aaaabababbbbaaaccccbbbbbbaadddd';

##</code><code>##

$s = '4ababa4b3a3a4c6b2a4d';

##</code><code>##

$s = 'aaaabababbbbaaaccccbbbbbbaadddd';
$s =~ s"($_{2,})"length($1).$_"ge for ('a'..'d');
print $s;