Um, wow, what tutorial are you reading?

I suggest starting with perlintro , esp Files and I/O and Simple matching and
perlfaq4#How do I find the first array element for which a condition is true?
perlfaq4#How do I process/modify each element of an array?
perlfaq5#How do I change, delete, or insert a line in a file, or append to the beginning of a file?
Re: split line from one point
Write to existing file with character insert

I also reccomend you read Tutorials: Context tutorial, because an array in scalar context returns the number of elements in the array -- not that you need an array for this task, because you don't, as all the examples above show

Also download a copy of Modern Perl and use it as a reference and see also http://perl-tutorial.org/