In no particular order...

• DBI - The sheer bliss when I converted our legacy flat-file base system to a *real* database.. ;)
• XML::LibXML - I can't do XML without this...
• Getopt::Long
• LWP
• Storable - so simple, so useful
• SOAP::Lite - just started using this, but it's going to save me a lot of time :)
• Date::Calc - cause I'm not smart enough to calculate dates
• Exporter
• IO - what would you do without it?
• Data::Dumper - I use it all the time to debug