Note that it won't work on references.

Or floating point numbers.

sub PI { 4 * atan2 1, 1 }
print "The value of PI is ${\~~PI}\n";
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Result:

The value of PI is 3

Ouch. That's a bit coarse.

The truth is: bitwise not only works on strings, and on integers. It works differently on both, though the net result of double not is pretty the same: restoration of the original string, or the original integer.