Net::SIP::Packet accepts as a string a valid SIP packet.
How a valid SIP packet looks like see RFC3261.
If you think you have a valid SIP packet which Net::SIP::Packet does not like
please post the full SIP packet (so that I may decide if it's a bug in Net::SIP
or if the packet is invalid) and also post the error you get from Net::SIP::Packet.
I'm not able to speculate about the problem without having these details.