I would start here: http://perldoc.perl.org/functions/time.html. Are you timing a specific program? If you are parsing logs, then you can read in the values and use a regular expression to pull out the corresponding numbers since you would know the output format. You can divide or multiply as necessary at that point.

The question is: are you trying to parse logs or time a specific script/program/function directly with Perl? We need to know what the input and output are to provide advice :).