See the current Perl documentation for cos.

Here is our local, out-dated (pre-5.6) version:

cos - cosine function

cos EXPR

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

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

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