Other options include using Win32::API and calling standard Win32 API functions, OR using Win32::OLE and WMI like so:

use strict;
use Win32::OLE;

my $WMI = Win32::OLE->GetObject("winmgmts:");
my $Procs = $WMI->InstancesOf("Win32_Process");

foreach my $Proc (in $Procs) {
    printf(
        "PID %u has %u threads.\n",
        $Proc->ProcessId,
        $Proc->ThreadCount,
    );
}
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Very quick & easy. The Win32_Process class has other valuable properties too. See the class documentation @ MSDN for more info.