sub two_greps {
        my %hash;
        @hash{@a} = (1) x @a;

        my @union        =  (@a, grep { ! exists $hash{$_} } @b);
        my @intersection =  (    grep {   exists $hash{$_} } @b);

}

Range (1,3) vs (0,9)
             Rate  one_cfor   one_for two_greps
one_cfor  50787/s        --      -20%      -25%
one_for   63371/s       25%        --       -7%
two_greps 68027/s       34%        7%        --
Range (1,5) vs (3,8)
             Rate  one_cfor   one_for two_greps
one_cfor  50813/s        --      -12%      -18%
one_for   57670/s       13%        --       -7%
two_greps 62150/s       22%        8%        --