Show us some sample code. Tell us the versions of Perl and and modules you are using. Tell us how much memory and virtual memory space are available.

If you can't write a succinct example the reproduces the issue, tell us how big the hash is that you are having trouble with.

Are you sure there aren't any differences in the type of data the large hash is storing compared with the small hash? Are you sure there aren't self referential loops in the large hash that don't exist in the small hash?