Perl5 リファレンス

       log EXPR

       log     Returns the natural logarithm (base e) of EXPR.
               If EXPR is omitted, returns log of `$_'.  To get
               the log of another base, use basic algebra: The
               base-N log of a number is equal to the natural log
               of that number divided by the natural log of N.
               For example:

                   sub log10 {
                       my $n = shift;
                       return log($n)/log(10);

               See also the exp entry elsewhere in this document
               for the inverse operation.