perldoc > sin(0)

    sin EXPR
    sin     Returns the sine of EXPR (expressed in radians). If EXPR is
            omitted, returns sine of $_.

            For the inverse sine operation, you may use the
            "Math::Trig::asin" function, or use this relation:

                sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }