Because you copied the referenced array's contents to a local copy and then modfied the local copy. You never copied it back. Try

use warnings;
use strict;

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

sub Foo
{
  my $refA = shift;
#  my @b = @$refA;
  $refA->[0] = "hello";
  $refA->[1] = "world";
  $refA->[2] = "Perl";
}
[download]