From the Win32::Process::Info documentation:

@pids = $pi->ListPids ();

    This method lists all known process IDs in the system. 
    If called in scalar context, it returns a reference to 
    the list of PIDs. If you pass in a list of pids, the 
    return will be the intersection of the argument list 
    and the actual PIDs in the system.

@info = $pi->GetProcInfo ();

    This method returns a list of anonymous hashes, each 
    containing information on one process. If no 
    arguments are passed, the list represents all processes 
    in the system. You can pass a list of process IDs, and 
    get out a list of the attributes of all such processes 
    that actually exist. If you call this method in scalar 
    context, you get a reference to the list.

    What keys are available depends on the variant in use. 
    You can hope to get at least the following keys for a 
    "normal" process (i.e. not the idle process, which is 
    PID 0, nor the system, which is some small 
    indeterminate PID) to which you have access:

        CreationDate
        ExecutablePath
        KernelModeTime
        MaximumWorkingSetSize
        MinimumWorkingSetSize
        Name (generally the name of the executable file)
        ProcessId
        UserModeTime
[download]

So you can use ListPids to get a list of all process id's and use GetProcInfo to associate each process id with a name.