Many functions and syntactic structures in Perl have default arguments. In most cases, this default argument is the variable $_. While this is a handy feature for experienced Perl programmers, it can make their code incomprehensible to those just learning the language. For novices, it can be a nuisance when one does not understand how the value of $_ is determined.
I recommend that when you are first learning Perl, put in all arguments explicitly. In many cases, Perl figures out what you are trying to do based on context and assigns values to $_ according to its own rules. The value of $_ can change subtly (or even drastically) depending on context.
Once you have a few lines of Perl under your belt and understand the ways of $_, feel free to use the default arguments. It is a nifty feature which allows you to write slick, fast, (and cryptic) Perl code.