As others say, they is no canonical answer, but FWIW here are some I end up using often

Data::Dumper # Oh yes, use this one a lot
DBI
LWP
JSON
POSIX
Time::HiRes
File::Find
Data::Dumper # did I mention that one yet ;)
[download]

Some you may want to avoid until you have learned more Perl, but most useful, and fun when you do have a need:

Threads
Thread::Queue
POE
[download]

Cheers,
R.