I hear you. But from what I can understand from the MSDN documentation the sequence of API calls required is :-

• take a snapshot of currently executing processes in the system using CreateToolhelp32Snapshot
• walk through the list recorded in the snapshot using Process32First and Process32Next, until we find the PID we want.
• get the first (and in this case only) thread details for list recorded in the snapshot using Thread32First
• use SetThreadPriority to modify the thread.

Looks like there is no direct access to the thread of the PID I want, but by reading thru the whole list we can get there.
I'm no expert, but the difficulty is getting Win32::API to let me make all the calls I need to. At the moment I'm getting error code 24 (incorrect length) on Process32First. My code :-

$pid = 952;
 # 952 is a running job pid
 use threads;
 use threads::shared;
 use Win32::API;
 Win32::API->Import( 'Kernel32', q[ DWORD GetLastError( ) ] );

$retcode = Win32::API->Import( 'Kernel32',
       q[ HANDLE CreateToolhelp32Snapshot
      ( DWORD dwFlags, DWORD th32ProcessID ) ] );
print "import CreateToolhelp32Snapshot code $retcode \n";

$hProcessSnap = CreateToolhelp32Snapshot(TH32CS_SNAPTHREAD, 0);
$retcode = GetLastError();
print "CreateToolhelp32Snapshot error code $retcode \n";
print "hProcessSnap = $hProcessSnap \n";

# typedef for PROCESSENTRY32
 typedef Win32::API::Struct AA => qw{
 DWORD dwSize;
  DWORD cntUsage;
  DWORD th32ProcessID;
  ULONG_PTR th32DefaultHeapID;
  DWORD th32ModuleID;
  DWORD cntThreads;
  DWORD th32ParentProcessID;
  LONG pcPriClassBase;
  DWORD dwFlags;
  TCHAR szExeFile[MAX_PATH];
 };

$retcode = Win32::API->Import( 'Kernel32', q[BOOL Process32First(     
+          
                 HANDLE hSnapshot,
                 LPAA lppe
               ) ] );
print "import process32first code $retcode \n";

$Point = Win32::API::Struct->new( 'AA' );
$Point->{dwSize}=Win32::API::Struct::sizeof( $Point );

Process32First($hProcessSnap, $Point);
$retcode = GetLastError();
print "process32first error $retcode \n";                

$retcode = Win32::API->Import( 'Kernel32', q[BOOL Process32Next(      
+         
                 HANDLE hSnapshot,
                 LPAA lppe
               ) ] );
print "import process32next code $retcode \n";

until ( $Point->th32OwnerProcessID == $pid ) {
         Process32Next($hProcessSnap, $Point);
         $retcode = GetLastError();
         print "process32next error code $retcode \n";            
}
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I get error 24 (incorrect length) on Process32First.