{
    my $r = substr rand, 2;
    sub crand() {
        length $r or ($r = rand) =~ tr/0-9//cd;
        chop $r;
    }
}

s[(.)]{$1 x (1+crand/3)}ge;

##</code><code>##

# ...
        length $r or ($r = rand) =~ tr/3-9//cd;
# ...

s[(.)]{$1 x crand/3}ge;