my %a1;
  undef @a1{@a1};
  my @in_both = grep { exists $a1{$_} } @a2;

##</code><code>##

  my (%a1, %a2);
  undef @a1{@a1};
  undef @a2{@a2};
  my @in_both = grep { exists $a1{$_} } keys %a2;