my $KeyPressed;
while (defined ($KeyPressed = $StdIn->InputChar(1)))
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It's possible to maintain the scoping of  $KeyPressed used in the OP code:
    while (defined(my $KeyPressed = $StdIn->InputChar(1))) {
        ...
        }

The reason the original code 'fails' is that a '0' character entered by the user evaluates as false, and the  while loop ceases iteration in consequence.