query about code

[Amichai Teumim]

Can someone explain to me what this script really does? I mean I see that it
lists dir within dir. But what is the code doing? For example all the blue
highlighted stuff, what is it doing?

#!/usr/bin/perl

$startdir = "/lib";

$level = 0;

list_dirs($startdir,$level);

sub list_dirs(){
 my $dir  = shift (@_);
 my $lev = shift (@_);


 opendir(TOP,$dir);
 my @files = readdir(TOP);
 closedir(TOP);

 shift(@files);
 shift(@files);

 foreach $file (@files){
   if(-d "$dir/$file"){
       spaces($lev);
       print "$file\n";
       list_dirs("$dir/$file",$lev+1);
   }
 }

}

sub spaces(){
 my($num) = shift(@_);
 for($i=0;$i<$num;$i++){
   print " ";
 }
}