Running your first script, the perl process hung/blocked on a syncronous read to the NPFS Win32 Named Pipe that is the cmd.exe process std handles.

     ntdll.dll!_KiFastSystemCallRet@0()     
     ntdll.dll!_NtReadFile@36()  + 0xc    
     kernel32.dll!_ReadFile@20()  + 0x67    
>    msvcr71.dll!_read_lk(int fh=0x00000005, void * buf=0x01ab870c, un
+signed int cnt=0x00000001)  Line 154 + 0x15    C
     msvcr71.dll!_read(int fh=0x00000005, void * buf=0x01ab870c, unsig
+ned int cnt=0x00000001)  Line 75 + 0xc    C
     perl512.dll!win32_read(int fd=0x00000005, void * buf=0x01ab870c, 
+unsigned int cnt=0x00000001)  Line 3746 + 0x12    C
     perl512.dll!PerlLIORead(IPerlLIO * piPerl=0x00285774, int handle=
+0x00000005, void * buffer=0x01ab870c, unsigned int count=0x00000001) 
+ Line 1049 + 0x11    C++
     perl512.dll!Perl_pp_sysread(interpreter * my_perl=0x00393f04)  Li
+ne 1711 + 0x34    C
     perl512.dll!Perl_runops_debug(interpreter * my_perl=0x00393f04)  
+Line 2049 + 0xd    C
     perl512.dll!Perl_call_sv(interpreter * my_perl=0x00393f04, sv * s
+v=0x00829f74, volatile long flags=0x0000000a)  Line 2605 + 0x36    C
     Glib.dll!gperl_closure_marshal(_GClosure * closure=0x00e025d0, _G
+Value * return_value=0x0006f888, unsigned int n_param_values=0x000000
+02, const _GValue * param_values=0x0006f8a8, void * invocation_hint=0
+x00000000, void * marshal_data=0x00393f04)  Line 105 + 0x96    C
     libgobject-2.0-0.dll!g_closure_invoke(_GClosure * closure=0x00e02
+5d0, _GValue * return_value=0x0006f888, unsigned int n_param_values=0
+x00000002, const _GValue * param_values=0x0006f8a8, void * invocation
+_hint=0x00000000)  Line 771 + 0x1b    
     libgobject-2.0-0.dll!io_watch_closure_callback(_GIOChannel * chan
+nel=0x00dcecf0, GIOCondition condition=G_IO_IN, void * data=0x00e025d
+0)  Line 94 + 0x15    
     libglib-2.0-0.dll!g_io_win32_dispatch(_GSource * source=0x00e0258
+0, int (void *)* callback=0x00b3c1c0, void * user_data=0x00e025d0)  L
+ine 979 + 0x1f    
     libglib-2.0-0.dll!g_main_dispatch(_GMainContext * context=0x00dc9
+698)  Line 2442 + 0xf    
     libglib-2.0-0.dll!g_main_context_dispatch(_GMainContext * context
+=0x00dc9698)  Line 3013 + 0x9    
     libglib-2.0-0.dll!g_main_context_iterate(_GMainContext * context=
+0x00dc9698, int block=0x00000001, int dispatch=0x00000001, _GThread *
+ self=0x00dc2ad8)  Line 3091 + 0x9    
     libglib-2.0-0.dll!g_main_loop_run(_GMainLoop * loop=0x00e3c2c8)  
+Line 3299 + 0x13    
     libgtk-win32-2.0-0.dll!gtk_main()  Line 1244 + 0x9    
     Gtk2.dll!XS_Gtk2_main(interpreter * my_perl=0x00393f04, cv * cv=0
+x0098af94)  Line 523    C
     perl512.dll!Perl_pp_entersub(interpreter * my_perl=0x00393f04)  L
+ine 2882 + 0x10    C
     perl512.dll!Perl_runops_debug(interpreter * my_perl=0x00393f04)  
+Line 2049 + 0xd    C
     perl512.dll!S_run_body(interpreter * my_perl=0x00393f04, long old
+scope=0x00000001)  Line 2308 + 0xd    C
     perl512.dll!perl_run(interpreter * my_perl=0x00393f04)  Line 2233
+ + 0xd    C
     perl512.dll!RunPerl(int argc=0x00000002, char * * argv=0x00283e08
+, char * * env=0x00284ed8)  Line 270 + 0x9    C++
     perl.exe!main(int argc=0x00000002, char * * argv=0x00283e08, char
+ * * env=0x00282c88)  Line 23 + 0x12    C
     perl.exe!mainCRTStartup()  Line 398 + 0xe    C
     kernel32.dll!_BaseProcessStart@4()  + 0x23
[download]

In Perl, the hang happened at

sub preview_call {
    my $buffer;
    my $rl = sysread($rfh,$buffer,1);<<<<<<<<<<<<<<<<<<<<<<<<<<
    if(defined($rl)){
        print "$rl + $buffer\n";
        while (Gtk2->events_pending()) {Gtk2->main_iteration();}
        return 1;
    }else{
        print "undefined\n";
        return 0;
    }
}
[download]

Long story short, select() on Win32, in C and in Perl, only works on sockets. Most POSIXy code assumes all FDs/handles are the same "type", so the authors never try to get Win32 async I/O models into their non-blocking POSIX code. You need to call PeekNamedPipe from kernel32 before doing a sysread() or read() from Perl. If the buffer has zero bytes, your read() will hang until some bytes are written to the pipe, or the other end of the pipe is closed.

Here is some untested code for calling PeekNamedPipe

use Win32API::File;
use Win32;
use Win32::API;
{
my $api;
die "PeekNamedPipe"
    if ! ($api= Win32::API::More->Import("kernel32", "
BOOL PeekNamedPipe(
  HANDLE hNamedPipe,
  LPVOID lpBuffer,
  DWORD nBufferSize,
  LPDWORD lpBytesRead,
  LPDWORD lpTotalBytesAvail,
  LPDWORD lpBytesLeftThisMessage
);"));
}
##########################
my ($TotalBytesAvail, $ret) = (0);
my $hnd = Win32API::File::FdGetOsFHandle($rfh->fileno()); #get Win32 k
+ernel handle from Perl land
if($hnd == Win32API::File::INVALID_HANDLE_VALUE()) {
    die "bad hnd";
}
$ret = PeekNamedPipe($hnd, undef, 0, undef, $TotalBytesAvail, undef);
if(!$ret) {
    my $err = Win32::GetLastError();
    die "PNP failed $err $^E";
 }
if($TotalBytesAvail) {
    #can safely read
}
[download]

Code is recycled from https://rt.perl.org/Ticket/Display.html?id=120330.

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