When the p5p guys added the system 1, ... asynchronous spawn, they wanted it to operate as much like the *nix equivalent as possible. Under *nix, a spawned process will hang around after it completes, as a zombie, until someone calls wait or waitpid to retrieve its exit status and clean it up.

Win32 didn't provide a direct equivalent of this facility, so they chose to emulate it within the Perl process. To do this, they store the process handles and pids in an internal data structure, so that the can emulate wait and waitpid.

    ## From win32.c
    if (mode == P_NOWAIT) {
    /* asynchronous spawn -- store handle, return PID */
    ret = (int)ProcessInformation.dwProcessId;
    if (IsWin95() && ret < 0)
        ret = -ret;

    w32_child_handles[w32_num_children] = ProcessInformation.hProcess;
    w32_child_pids[w32_num_children] = (DWORD)ret;
    ++w32_num_children;
    }
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However, the mechanism they chose to use for the wait & waitpid emulation has a limit of 64 elements, so when you try to spawn a 65th process, without having called wait or waitpid in the interim, they reject the attempt because the fixed sized tables (internal to the Perl process) have temporarially run out of resources.

    case P_NOWAIT:  /* asynch + remember result */
    if (w32_num_children >= MAXIMUM_WAIT_OBJECTS) {
        errno = EAGAIN;
        ret = -1;
        goto RETVAL;
    }
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So, if you need to start more than 64 processes this way, you will need to clean up your zombies using wait as you go.

However, there is another alternative built-in. usage: Win32::Spawn($cmdName, $args, $PID).

C:\test>perl -le "for (1..100) {Win32::Spawn( $ENV{ComSpec},'/c echo>n
+ull', $pid );print qq[$_\tpid:$pid\t],$?==-1 ? $! : 'OK' };"
1       pid:5492        OK
2       pid:5256        OK
3       pid:1860        OK
...
93      pid:2236        OK
94      pid:2308        OK
95      pid:4012        OK
96      pid:4796        OK
97      pid:4636        OK
98      pid:4780        OK
99      pid:244 OK
100     pid:4800        OK
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This doesn't attempt to emulate *nix mechanisms, and so doesn't have any limitations that arise from doing so. It also returns the real pid directly which can be useful.