Hello hankcoder,

I'm not quite understand what is $_[0]

The special variable @_¹ is documented as follows:

Within a subroutine the array @_ contains the parameters passed to that subroutine. Inside a subroutine, @_ is the default array for the array operators pop and shift.

Now, in Perl² you access the individual elements of array @my_array as $my_array[0], $my_array[1], $my_array[2], .... (Note that the first element has index zero.) In the same way, the array @_ (which is named “ _ ”) has elements $_[0], $_[1], $_[2], .... So $_[0] is the first parameter passed to the subroutine.

¹You can also call it @ARG if you use English; — see perlvar#General-Variables.
²That is, Perl 5 and earlier. I believe Perl 6 uses a different syntax.

Hope that helps,