In no particular order:

1. Fatal
2. File::Spec
3. Digest::MD5
4. Time::Piece (ok, so I'm biased)
5. DBI
6. CookBookA && CookBookB (not really modules, but indispensible to anyone learning XS)
7. POE
8. Log::Dispatch::Config
9. XML::SAX::Machines
10. XML::Handler::AxPoint (biased again)