sub random_pick {
  my ( $n, $arr ) = @_;
  my @idx = 0..$#$arr;
  my $i = 0;
  while ( $i < $n ) {
    my $j = $i + rand( @idx - $i );
    @idx[ $i, $j ] = @idx[ $j, $i ];
    ++$i;
  }
  return @{ $arr }[ sort { $a <=> $b } @idx[ 0 .. $n-1 ] ];
}