This script is an offshoot of the original weather script included with IRSSI, which no longer works. It used the Geo::Weather, which is no longer maintained.
Weathercom.pl uses the well-written Weather::Com module. You must register an account with weather.com to use it. It requires a "partnerID" and "licenseID". Download here
To use, download it and then put the script into ~/.irssi/scripts. Then add your settings to ~/.irssi/startup:
/run weathercom
/set weather_licenseKey 451b7f84522d5abc6b9
/set weather_partnerId 5548889210
/set weather_location 94598