Just noting down a small caveat that using unary you may loose a bit of numeric range in Perl (including this method). On a 64 machine the valid range for this method would be from -(2^63): -9223372036854775808 to (2^63-1) 9223372036854775807 (thus signed 64 bit integer). Positive Perl integers can be bigger but the result of the unary overlaps the negative range for positive numbers in the upper range (> 2^63-1).

To illustrate this with some numbers:

 ~ -9223372036854775808 = 9223372036854775807
 ~  9223372036854775808 = 9223372036854775807
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I have not checked all of the other (now 13) proposed solutions, but just spotted this one. I think there are proposed solutions that do not suffer from this (such as Discipulus's method).