Straight from the CGI::Cookie man page
in reply to Re: Recommended modules for handling cookies?
in thread Recommended modules for handling cookies?
This is a partial or complete domain name for which
the cookie is valid. The browser will return the
cookie to any host that matches the partial domain
name. For example, if you specify a domain name of
".capricorn.com", then Netscape will return the cookie
to Web servers running on any of the machines
"feckless.capricorn.com", etc. Domain names must
contain at least two periods to prevent attempts to
match on top level domains like ".edu". If no domain
is specified, then the browser will only return the
cookie to servers on the host the cookie originated