| Subject: | ARG1 contains part message |
According to the doc ARG0 of the on_part event is the hostmask of the parting user, and ARG1 contains the channel name. Unfortunately though if a user parts with a message:
eg: * @MrPilot has left #Channel (cycling)
ARG1 will contain (as reported by Data::Dumper): $VAR1 = '#Channel :cycling';