Perl uses doubles for floating point numbers. Doubles have 52 bits of precision, which translates to roughly 15 (log₁₀ 2⁵²) digits of precision. Therefore, an error on the 16^th significant digit shouldn't be unexpected when converting the string to a number. Remember, to convert 0.4079... from a string to a number, the computer has to do something like 4*10^-1 + 7*10^-3 + 9*10^-4 + 6*10^-5 + ... and some numbers (such as 0.4, IIRC) are periodic numbers in binary.