sub two_greps {
my %hash;
@hash{@a} = (1) x @a;
my @union = (@a, grep { !$hash{$_} } @b);
my @intersection = ( grep { $hash{$_} } @b);
}
__DATA__
Range (1,3) vs (3,9)
Rate one_cfor one_for two_greps
one_cfor 55648/s  18% 23%
one_for 68027/s 22%  5%
two_greps 71891/s 29% 6% 
Range (1,5) vs (3,8)
Rate one_cfor one_for two_greps
one_cfor 50378/s  13% 15%
one_for 58140/s 15%  2%
two_greps 59277/s 18% 2% 
Update: the reason for the error is that = has a higher precedence than ,. Therefore the bad version was like doing: (my @union = @a), grep { !$hash{$_} } @b;. The grep was executed, but the results were discarded.
A more serious error with the greps is that we test the value in the hash, not its existence. So 0 would never be in the intersection of a set, and always in the union. Two greps still wins by a small amount after that is fixed, too.
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% 
