use Benchmark;

$build = <<'BUILD';
my @a = (0..10000);
my @b = (9000..19000);
BUILD

$iterate = <<'ITERATE';
my @a = (0..10000);
my @b = (9000..19000);
my (@matches,@mismatch);
@matches = @mismatch = ();
foreach $a (@a) {
   push @matches, $a if grep{$a == $_}@b;
}
foreach $a (@a) {
   push @mismatch, $a unless grep{$a == $_}@b;
}
ITERATE

$hash = <<'HASH';
my @a = (0..10000);
my @b = (9000..19000);
my (@matches,@mismatch);
@matches = @mismatch = ();
map{$seen{$_}++}@b;
@matches  = grep{defined $seen{$_}}@a;
@mismatch = grep{!defined $seen{$_}}@a;
HASH

timethese(1,{
    'build'     =>  $build,
    'iterate'   =>  $iterate,
    'hash'      =>  $hash
    });

__END__
C:\>perl test.pl
Benchmark: timing 1 iterations of build, hash, iterate...
     build: -1 wallclock secs ( 0.00 usr +  0.00 sys =  0.00 CPU)
            (warning: too few iterations for a reliable count)
      hash:  0 wallclock secs ( 0.16 usr +  0.00 sys =  0.16 CPU) @  6.25/s (n=1
)
            (warning: too few iterations for a reliable count)
   iterate: 147 wallclock secs (147.20 usr +  0.00 sys = 147.20 CPU) @  0.01/s (
n=1)
            (warning: too few iterations for a reliable count)

C:\>