Hi litebrite! Eduardo's answer is quite good when he talks about the reference to a vagina. 'Perereca', a sort of small frog, is one of the many names given to a vagina in popular oral language. So, 'perereca sem dono' means literally 'vagina ownerless' (actually something like 'pussy ownerless'). 'Dono', in this case, means owner. 'Sem', you probably know, means 'without'.
I cannot tell you anything about this 'carnival block' he mentioned, I'm not from Rio. However, There are so many carnival organized groups in this city that use double meaning names to identify themselves, often related to sex matters.