You are going about that completely the wrong way.

In breif, when you are testing the lines seperately, the chances are that the second line does almost nothing after having been repeated one or two times as there is nowhere left to lookdown() to.

However, when you repeat the two lines together, you are re-parsing the file each time, and so the tree is reset and lookdown() starts at the top and has some work to do every iteration.

Basically, you cannot profile code that has state by repeating it over and over without reseting the state each time.

The answer is to use the right tool for the job. Ie. Profiling and not Benchmarking. Get yourself over to CPAN and grab Devel::Smallprof. It's 10 times easier and a thousand times more accurate than the method your trying right now.