My interpration of your question is somewhat different from the responses posted thus far - it looks like you want to modify %innerhash, after saving a "reference" to it, and you fear (correctly) that the reference will also change.

The answer is that what you save should be a COPY, not a reference.

If the %innerhash is only one level deep (i.e. it is shallow), you can save a copy like this:

  $hash {somekey} = {%innerhash}; # Shallow copy
[download]

Now, you are free to modify %innerhash, and $hash{somekey} will maintain it's original value.

IF %innerhash is more than one level deep, you will need to do a "deep copy" - search for that at this site if you need it.