assigns a new reference (by creating a new anonymous array) to $arr[$i] replacing the reference copied from @_. Without that reference you can't change the contents of @_

Whereas @{$arr[$i]} = reverse @{$arr[$i]} uses the reference copied from @_ to change the existing array elements.

# $#_[$i] doesn't work

sub print2DArray {
  for my $i (0 .. $#_) {  
    for my $j (0 .. $#{$_[$i]}){
      print $_[$i][$j]," ";
    }
    print "\n";
  }
}
[download]

sub print2DArray7 {
  for (@_) {  
    print join " ",@$_;
    print "\n";
  }
}
[download]