Hi Rob,

Sorry, I have no idea about Perl (I'm on Delphi Pascal). About Canon ISO...
For all cameras, Canon writes two ISO values inside Makernotes: AutoISO and BaseISO tag. For most cameras, there's also ISO written into Exif, which is calculated from AutoISO and BaseISO value. This is always the case for dslr cameras -for pocket camera, it depends on camera settings (manual or auto).
I'm quite sure Phil will come around, to give you a hint for Perl...

Greetings,
Bogdan