Interprets EXPR as a hex string and returns the corresponding value.
(To convert strings that might start with either 0, 0x, or 0b, see
/oct.)  If EXPR is omitted, uses $_.
    print hex '0xAf'; # prints '175'
    print hex 'aF';   # same
Hex strings may only represent integers. Strings that would cause integer overflow trigger a warning. Leading whitespace is not stripped, unlike oct().