Note that $pythonCmd to which you have assigned a value is not the same as $pythonCMD which you have used in your call to open(). If you had used strict this would have been reported to you because $pythonCMD would not have been declared. But your code won't compile anyway because the print statement inside Validate() is unterminated.

As this is therefore likely not the code which you are actually running any advice would be purely guess work. My guess is you are Suffering from Buffering.