++ for two useful and important points — 1’s vs. 2’s complement, and negation leaving a number unchanged (!) — but I have to disagree about the terminology. “Bitwise complement” is ambiguous for precisely the reasons you give, unless “1’s complement” is explicitly specified. But “bitwise negation” is unambiguous, and is the term used in the official documentation (perlop#Symbolic-Unary-Operators):
Unary "~" performs bitwise negation, that is, 1's complement. For example, 0666 & ~027 is 0640. ... Note that the width of the result is platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 bits wide on a 64-bit platform, so if you are expecting a certain bit width, remember to use the "&" operator to mask off the excess bits.
The Camel Book refers to ~ as “bitwise NOT” (4th Edition, p. 118).
Happy Christmas,
