#!/usr/bin/perl -wl

    use strict;


    my $s = 'aaaabababbbbaaaccccbbbbbbaadddd';

    print $s;

    $s =~ s/((.)\2+)/length($1) . $2/eg;

    print $s;


    __END__
    Prints:
    aaaabababbbbaaaccccbbbbbbaadddd
    4ababa4b3a4c6b2a4d