Maybe you could try the lwp-download script instead... because

"The lwp-download program is implemented using the libwww-perl library. It is better suited to download big files than the lwp-request program because it does not store the file in memory. Another benefit is that it will keep you updated about its progress and that you don't have much options to worry about."

If that doesn't work either, then there's also other (non-Perl) tools, such as curl.

(In cases like these, just trying different tools often helps to narrow down on the problem. For example, if curl would also stop at 78 MB, then it's likely the problem lies outside of either of those two tools.)