You can generalize it for any number of repeated characters too.

while ( my $string = <DATA> ) {
    $string =~ s/(.)(\1+)/$1.length($1.$2)/ge;
    print $string;
}

__DATA__
Balloon
bookkeeper
Wheeeee!!!
[download]

Output:

Bal2o2n
bo2k2e2per
Whe5!3
[download]