SOAP::Serializer stores namespaces in a hash ref with namespaces as keys and prefixes as values.
This is broken by design: It does not allow multiple prefixes to be assigned to one namespace, and does not prevent assigning the same prefix twice.
The namespace handling should be reversed: prefixes should be used as keys and namespaces as values.
For prefix-resolving, reversing the hash is OK, and for registering a new prefix, the prefix hash can be checked with exists().
The default prefixes used must check the prefix hash before registering, too (and resort to alternatives, if they are already taken).