Because operators == and != force numeric context on their operands.

Add use warnings to your code and it will start working properly

Use of uninitialized value in numeric ne (!=) at w.pl line 8.
Argument "Hi" isn't numeric in numeric ne (!=) at w.pl line 8.
Use of uninitialized value in numeric ne (!=) at w.pl line 9.
Argument "" isn't numeric in numeric ne (!=) at w.pl line 9.
Use of uninitialized value in numeric ne (!=) at w.pl line 10.
Use of uninitialized value in numeric ne (!=) at w.pl line 11.
$number is defined
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