use Benchmark qw(:all) ;

my $test_element = 42_000;
my @a = (1..100_000);

my $count = 20;
cmpthese($count, {
                  'hash' => sub {
                     my %seen;
                     undef( @seen{ @a } );  # a trick I learned from tilly
                     my $found = 0;
                     for my $iter (10_000..10_100) { # 101 searches
                        $found = 1 unless exists $seen{ $iter };
                     }
                     return $found;
                  },
                  'for' => sub {
                     my $found = 0;
                     for my $iter (10_000..10_100) { # 101 searches
                        foreach (@a) {
                           $found = 1 and last if $iter == $_;
                        }
                     }
                     return $found;
                  },
                 });

##</code><code>##

Benchmark: timing 20 iterations of for, hash...
       for: 16 wallclock secs (15.67 usr +  0.00 sys = 15.67 CPU) @  1.28/s (n=20)
      hash:  8 wallclock secs ( 8.57 usr +  0.04 sys =  8.61 CPU) @  2.32/s (n=20)
       Rate  for hash
for  1.28/s   -- -45%
hash 2.32/s  82%   --