Win32::Mutex sounds like a simple and appropriate solution. If you end up using File::Flock, beware of the case where a file exists after an abnormal termination of a process. That is, where there is a file and no process.

Yet another approach is to use Tie::Registry to store the process id of the process in the Registry when it starts. The pseudo code would be something like this:

if the key is in the registry and
  that process (value of the key) is still running, exit
put the processid of the current process in the registry,
    creating the key if necessary
"rest of program"
[download]

HTH, --traveler