I like how this place works! I can combine suggestions from Roger and -Mouse to get a window only for STDERR:

package ouse;

BEGIN {
   use IO::Capture::Stderr;
   $ouse::capture = IO::Capture::Stderr->new();
   $ouse::capture->start();
}

END {
   $ouse::capture->stop();
   my @err = $capture->read;
   if ($#err >= 0) {  
     print "$_" for @err;
     <STDIN>;
   }
}

1;
[download]

Thank you for taking the time.