I would like to second many of the choices made by other monks. However I would like to recommend a few more that no one else seems to have mentioned:

Tk - for creating graphical user interfaces.

GD and GD::Graph - For manipulating graphical objects

Bioperl - For dealing with DNA sequences.

Compress::Zlib and its more user friendly daughter Archive::Zip for working with compressed files.

-- Regards,
Helgi Briem
helgi AT decode DOT is