A couple of modules I couldn't live without (well, almost):

File::Basename - Neat little module for picking apart a path/filename/suffix without having to do it with a regex.
File::Copy - Nice to be able to copy and move files without having to resort to system() calls.
File::Path - mkdir() doesn't make paths recursively, this does.
File::Slurp - I don't write my own read_file() subs anymore. :-)
HTML::Template - Fast and simple templating.
MIME::Lite - Send multipart emails easily.
XML::Twig - When the XML document is too large to slurp in all at once.