My thanks Perrin and Derby,
I appreciate you bothering to look into this on my account, I'm annoyed I didn't look further and spot it myself.
My fix is simply the below, inside the &build_subs() loop:

next if exists &$_;
[download]

It appears to work, so I'm hoping this is the correct course of action.

Thanks again Monks,
JP

-- Alexander Widdlemouse undid his bellybutton and his bum dropped off --