Please do not post your homework questions here.

Creating a program to go through log files is very, very easy. What parts have you written and where exactly do you have a problem?

A good approach to any unknown problem is to first solve it a few times as a human. Then take a step back and find out what steps you took as a human to solve the problem. The last step is then to encode these steps in a program.

For going through a log file, for example, I would read it line by line and mark or write down all entries of interest.

See perlintro, and The Diamond Operator and perlsyn for if statements.