If your script runs under Apache, then I would use $ENV{UNIQUE_ID}. Otherwise Data::UUID should do what you need. Here is an excerpt from its man page:

This module provides a framework for generating UUIDs (Universally Unique Identifiers, also known as GUIDs (Globally Unique Identifiers). A UUID is 128 bits long, and is guaranteed to be different from all other UUIDs/GUIDs generated until 3400 A.D. UUIDs were originally used in the Network Computing System (NCS) and later in the Open Software Foundation's (OSF) Distributed Computing Environment. Currently many different technologies rely on UUIDs to provide unique identity for various software components, Microsoft COM/DCOM for instance, uses GUIDs very extensively to uniquely identify classes, applications and components across network-connected systems.

HTH, Valerio