I did get the code to work.. seems to me that details of threads of process x can indeed be changed from within process y - code follows :-

# set first thread for $pid to $hThreadPriority
my $pid = 952;
my $hThreadPriority = 0;
my $retcode;
my $hThreadSnap;
my $hThread;
my $Point;

 use Data::Dumper;
 use strict;
 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";

use constant TH32CS_SNAPTHREAD => 0x4;

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

# typedef for THREADENTRY32
typedef Win32::API::Struct TE32 => qw{
  DWORD dwSize;
  DWORD cntUsage;
  DWORD th32ThreadID;
  DWORD th32OwnerProcessID;
  LONG tpBasePri;
  LONG tpDeltaPri;
  DWORD dwFlags;
  };

$retcode = Win32::API->Import( 'Kernel32',
                   q[ BOOL Thread32First
                   ( HANDLE hSnapshot, LPTE32 lpte)]);
print "import thread32first code $retcode \n";

$Point = Win32::API::Struct->new( 'TE32' );
$Point->{dwSize}=28;

Thread32First($hThreadSnap, $Point);
$retcode = GetLastError();
print "thread32first error $retcode \n";
print "pid = $Point->{th32OwnerProcessID} \n";

$retcode = Win32::API->Import( 'Kernel32',
                   q[ BOOL Thread32Next
                   ( HANDLE hSnapshot, LPTE32 lpte)]);
print "import threadnext code $retcode \n";


until ( $Point->{th32OwnerProcessID} == $pid ) {
         Thread32Next($hThreadSnap, $Point);
         $retcode = GetLastError();
         print "thread32next error $retcode \n";
         print "pid = $Point->{th32OwnerProcessID} \n";
}


$retcode = Win32::API->Import( 'Kernel32', q[HANDLE OpenThread(
               DWORD dwDesiredAccess,
               BOOL bInheritHandle,
               DWORD dwThreadId
               ) ] );
print "import openthread code $retcode \n";

use constant THREAD_SET_INFORMATION => 0x0020;
use constant THREAD_QUERY_INFORMATION => 0x0040;

$hThread = OpenThread(
              THREAD_SET_INFORMATION|THREAD_QUERY_INFORMATION,
              0,
              $Point->{th32ThreadID}
              );
$retcode = GetLastError();
print "openthread error code $retcode \n";

$retcode = Win32::API->Import( 'Kernel32', q[BOOL SetThreadPriority(
               HANDLE hThread,
               int nPriority
               ) ] );
print "import openthread code $retcode \n";

SetThreadPriority(
              $hThread,
              $hThreadPriority
              );
$retcode = GetLastError();
print "setthreadpriority error code $retcode \n";
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