Even though I don't understand what "for my $i (0 .. $#arr)"

$#array is the index of the last element of @array. Which is the same as the scalar(@array)-1;

a subroutine can change variables from the function it was called from

Yes, if you modify @_, it changes for the caller.

$#array is the index of the last element of @array. Which is the same as the scalar(@array)-1;

Depending on which version of perl and the value of $[.     :)

What I don't understand is why I get different results in this code:

use strict;
use warnings;


problem([1,2,3]);

sub problem {
    print2DArray(@_);  #Output: 1 2 3
    reverseArray1(@_);
    print2DArray(@_);  #Output: 1 2 3
    reverseArray2(@_);
    print2DArray(@_);  #Output: 3 2 1
}

sub reverseArray1 {
    my @arr = @_;
    for my $i (0 .. $#arr) {
        $arr[$i] = [reverse @{$arr[$i]}];
    }
}

sub reverseArray2 {
    my @arr = @_;
    for my $i (0 .. $#arr) {
        @{$arr[$i]} = reverse @{$arr[$i]};
    }
}

sub print2DArray {
    for my $i (0 .. $#_) {  # How does that work for the nested for lo
+op?
        for(my $j=0;$j<scalar(@{$_[$i]});$j++){  # $#_[$i] doesn't wor
+k
            print $_[$i][$j]," ";
        }
        print "\n";
    }
}
[download]

Whats the difference between

$arr[$i] = [reverse @{$arr[$i]}];
[download]

and

@{$arr[$i]} = reverse @{$arr[$i]};
[download]

?
I still am quite new to perl, and I appreciate all the comments!

$arr[$i] = [reverse @{$arr[$i]}];

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]

or

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

poj

... I don't understand what "for my $i (0 .. $#arr)" means ...

The other point to remember about this type of loop (a so-called Perl-style for-loop) is that is "topicalizes" (see "topic" in perlglossary)  $i or whatever variable you specify, or implicitly  $_ if you don't explicitly specify any variable, to each element of the loop list; see Foreach Loops in perlsyn. Also note that foreach and for are exactly synonymous and completely interchangeable keywords in Perl; the differing behavior of Perl- and C-style for/foreach-loops is determined by the syntax of the loop list expression.