Hi, I am new to Perl. I've got a task to convert a given time (e.g, 6 minutes 30 secs or 24 secs 50 millisecs) completely in seconds.

Welcome to the Monastery Lucifer.
The problem with your question is what I refer to as "problem definition." What format is your input being resented as (eg. 00:00:00.000 would be one way).

While you are at it check out Understanding and Using Perl Monks so that you can effectively ask questions that yield useful answers.