Sorry I haven't looked back for a while - I did see the first few comments, but now there are more. It's suprising what we can learn from each other!

Anyway, I didn't make my initial question clear enough (my mistake, sorry): None of the data structures in data_defs will change (I hope!), and will be shared by all of the other modules.

I would also like to make them 'global' - I tried using our for them but the only way I can make it work is to use this at the top of each file that needs them (although there are only a couple):

require 'data_defs.pl';
use vars qw/ ... /
[download]

where '...' are the structures in data_defs.pl.
I have also tried making data_defs a USEable module, but I'm not sure which would be best for what I want.
This namespace stuff is confusing the hell out of me!

Anyway, thank you all very much for your help.
Soooo much still to learn, but I love learning and love Perl even more. What a wonderful language!