Perhaps you've hit a bug:

1.08 - 6 Jan 2000 * uncompress was clobbering its input parameter. Now it doesn't. This bug was spotted by Deven T. Corzine.

Perhaps clobbering the input ref causes the error ? I guess you could try either passing a regular scalar, or upgrading to version 1.11 of Compress::Zlib, if you haven't already.