Start by searching CPAN and Google, to see how much of the wheel you need not reinvent! My search turned up these resources:

• CPAN modules Net-YMSG and Net-YahooMessenger; Neither looks complete, but you might complete them, or use them as a starting point for your module, or just steal their docs.
• Gaim is a very popular instant messaging client, that handles (among many others) the Y! Messenger protocol, and allows direct Perl scripting.

I see that you received similar advice several months ago in a previous thread. Have you asked either of the module authors about taking over maintenance for an existing module? Have you tried Gaim's Perl scripting?

If you *must* forgo the simpler avenues of attack, then consider writing your module as an Inline::C interface to libgaim. At the very least, try to glean the protocol info from the Y! Messenger sections of the Gaim source code. Packet sniffing can be fun, but not when the traffic you are trying to decipher has been deliberately obfuscated.