I am dead positive. When I do it with 10 URLS, it works 100% of the time. 50 URLS and it still works. But when I get higher, say 100 or so, it works 1/3 of the time or so. No errors, it just just post the message.
Hmm... It could be a bug in WWW::Mechanize, but a quick scan of the bug reports doesn't show this being reported before (though I might have missed it). It might also be a server fault. Have you tried posting the same data via another mechanism (like, via a browser)?
update: alternatively, can you make a smallish program that demonstrates the problem?