How?? Didn't I pass a reference?

That you did, but you also copied the contents of the array that reference pointed to into a new array, which as you can guess, isn't going to effect the array that said reference is pointing to. Here's what you really want

use warnings;
use strict;

my @a = ("a", "b", "c");
Foo(\@a);
print "$_ " foreach(@a);

sub Foo
{
  my $refA = shift;
  ## dereference the array
  $refA->[0] = "hello";
  $refA->[1] = "world";
  $refA->[2] = "Perl"; 
}

__output__

hello world Perl
[download]

It might be a good idea to learn more about references and how they work in perl. See. perlreftut, perlref, perldsc and tye's References quick reference for further information.
HTH

_________
broquaint