cos EXPR
    cos     Returns the cosine of EXPR (expressed in radians). If EXPR is
            omitted, takes the cosine of $_.

            For the inverse cosine operation, you may use the
            "Math::Trig::acos()" function, or use this relation:

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