If all you need is that one function, you can easily get at it with Inline::C:

#! perl -slw
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => 'GWTPI', CLEAN_AFTER_BUILD =>0;
#include <windows.h>

#define IS_VARS Inline_Stack_Vars
#define IS_RESET Inline_Stack_Reset
#define IS_PUSHIV( iv ) Inline_Stack_Push( sv_2mortal( newSViv( iv ) )
+ )
#define IS_PUSHUV( uv ) Inline_Stack_Push( sv_2mortal( newSVuv( uv ) )
+ )
#define IS_DONE Inline_Stack_Done

void getWindowThreadProcessId( SV* hwnd ) {
    IS_VARS;
    DWORD pid;
    DWORD tid = GetWindowThreadProcessId( (HWND)SvUV(hwnd), &pid );

    IS_RESET;
    IS_PUSHUV( tid );
    IS_PUSHUV( pid );
    IS_DONE;
    return;
}
END_C

my( $tid, $pid ) = getWindowThreadProcessId( 0 );
print "$tid $pid";
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