You have a number of errors in your grammar:

1. In the rule ip_nosemi, you forgot to backslash the last "</code>\d</code>".
2. In hosts_or_range, you have host(s?), when you probably meant to put the (s?) in network object.
3. You can't use $item as both a hash and an array in the same production!

I also reorganized your grammar a bit. It should work now, although I'm on a machine without PRD installed at the moment so you shouldn't just take my word on that. However, tachyon is right on this one; PRD is far too large of a hammer to use parse data like this. :)

startrule:       network_object(s)
network_object:  /network:(\w+)/ '=' '{'
                     'ip'   '=' ip
                     'mask' '=' ip
                     hosts_or_range(s?)
                 '}'
                 { print "Found network object named $item[1] ", 
                         "with IP $item[6] and MASK $item[9]\n"
                 }

hosts_or_range:  host | host_range
host_range:      /host:\w+/ '=' '{' 'range' '=' ip '-' ip    '}'
host:            /host:\w+/ '=' '{' 'ip'    '=' ip(s) '}'
ip:              / \d{1,3} (?:\.\d{1,3}){3} ;? /x
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