You are viewing the version of this documentation from Perl 5.33.2. This is a development release of Perl.
cos EXPR

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] ) }