sub fisher_yates_shuffle {
  my $deck = shift;    # $deck is a reference to an array
  my $i    = @$deck;
  while ( $i-- ) {
    my $j = int rand( $i + 1 );
    @$deck[ $i, $j ] = @$deck[ $j, $i ];
  }
}

my @aa = qw/a b c d e f/;
fisher_yates_shuffle( \@aa );