Maybe this will explain the save&restore and context caching mechanism better than I've achieved using words:

#! perl -slw
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => '_867652', CLEAN_AFTER_BUILD => 0;


PerlInterpreter *saved;
SV *callback;

VOID CALLBACK cbProc( DWORD time ) {
    PerlInterpreter *temp = Perl_get_context();

    printf( "C-Callback: %u\n", time );

    PERL_SET_CONTEXT( saved );
    {
        dSP;
        ENTER;
        SAVETMPS;
        PUSHMARK( SP );
        XPUSHs( sv_2mortal( newSVuv( (UV)time ) ) );
        PUTBACK;

        call_sv( callback, G_DISCARD );

        FREETMPS;
        LEAVE;
    }
    PERL_SET_CONTEXT( temp );
    return;
}

void __cdecl thread( VOID *arg ) {
    while( Sleep( 5000 ), 1 ) {
         printf( "Here\n" );
         cbProc( GetTickCount() );
     }
}

void setCallback( SV* cv ) {
    dTHX;
    saved = Perl_get_context();
    callback = cv;

    // Start callback timer thread
    _beginthread( &thread, 0, NULL );
    return;
}

END_C

$|++;

sub callback {
    print "Timer value is: $_[0]";
    return;
}


setCallback( 'callback' );

sleep 100;

__END__
C:\test>867652
Here
C-Callback: 1355573480
Timer value is: 1355573480
Here
C-Callback: 1355578472
Timer value is: 1355578472
Here
C-Callback: 1355583479
Timer value is: 1355583479
Terminating on signal SIGINT(2)
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