If you have both instances running on the same machine at the same time, take a look a using a tied-hash (particularly if there are more than one variable involved).

If you have successive executions, then you can read and write the state to a configuration file. (See CPAN for your choices here.)

If you have instances running at the same time on different machines, the co-ordination problem gets tougher. Sockets will come into play and a lot of work to verify that your 'global variables' really are in sync across the Network.

If the globals are read-only, I'd be tempted to design a forked master/worker solution, where the globals are set by the master, possible by reading a state-file, before forking off a worker task to complete the process.

Which ever way you choose, there will be issues of synchrony and update consistancy that you will have to address.

If you are running two instances seperated in time (one after the other) then XML::Simple and Storable are usefull.

Or, have both programs use the same module, and the global variable is defined there.

What are you trying to accomplish?