*;$ is the prototype definition of this subroutine and you can read all about it through the links already quoted above.

However, don't start using prototypes everywhere. They are rarely needed and in many places unneccessary or even wrongly used.

So the main rule is: don't use prototypes unless you certainly know what you are doing. And its use is NOT about checking the type of the sub's parameters.

In all honesty I can say that in all the years I have been programming Perl, I may have used prototypes once or twice only and I'm not entirely sure they were really necessary.