I reviewed the thread you're speaking of. The thread dates back to the year 2005. The module is actively maintained, and while it's possible this was never reported/fixed, it's also probable that this has already been dealt with.

Also, in 2005 the average machine didn't have nearly so much memory as nowadays. If the issue was a memory constraint, it's possible that too has been resolved simply by the application of Moore's Law over the past seven or eight years. Older versions of Perl such as 5.6.2 have mostly fallen by the wayside too, and 5.6, IIRC, was more memory constrained than modern versions.

Or you could be screwed, in which case breaking the download down to under 20k messages seems like a reasonable workaround. But why not code it the way you think you want it to behave (ie, don't worry about breaking down to 20k messages per iteration), and then if you bump into the problem, be ready to report the bug, and/or to code around it.