If we are assuming that isook() is fine functionally, there is a much faster way. Don't use regex! use substr! I ran same benchmarks along with a new one, substr(). Code is below. My machine, (older Win XP running Perl 5.10) is considerably slower than the other machines posting results, but I would expect similar results in the ranking if you run the code below. Having said that, all these solutions run so fast that it is hard to see how this is going to make much difference, but that depends upon the application!

This substr idea is also faster even with some simple format error tests like this snippet... This $1,$2,$3 stuff is "expensive".

substr => sub{
      my $t2 = '20090123';
      my $year =substr ($t2,0,4); 
      my $mon = substr ($t2,4,2);
      my $day = substr ($t2,6,2); 
      die if (length($t2)!= 8);
      die if ($t2 =~ /\D/);
    },
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#!perl
use v5.10;
use strict;
use warnings;
use Benchmark qw(:all);

my $results = timethese( 1e6,
  {
    repeat => sub{
      my $t1 = '20090123';
      $t1 =~ /(\d\d\d\d)(\d\d)(\d\d)/;
      my ($y1,$m1,$d1) = ($1,$2,$3);
    },
    range => sub{
      my $t2 = '20090123';
      $t2 =~ /(\d{4})(\d{2})(\d{2})/;
      my ($y2,$m2,$d2) = ($1,$2,$3);
    },
    substr => sub{
      my $t2 = '20090123';
      my $year =substr ($t2,0,4); 
      my $mon = substr ($t2,4,2);
      my $day = substr ($t2,6,2); 
    },
    chkunpk => sub{
      my $t3 = '20090123';
      $t3 =~ m/([0-9]{8})/;
      my ($y3,$m3,$d3) = unpack "A4 A2 A2", $1;
    },
    dirunpk => sub{
      my $t3 = '20090123';
      my ($y4,$m4,$d4) = unpack "A4 A2 A2", $t3;
    },
    isook => sub{
      my $t5 = '20090123';
      $t5 =~ /(....)(..)(..)/;
      my ($y5,$m5,$d5) = ($1,$2,$3);
    },
  } );

cmpthese( $results ) ;

__END__
output:
Benchmark: timing 1000000 iterations of chkunpk, dirunpk, isook, range
+, repeat, substr...
   chkunpk:  4 wallclock secs ( 5.05 usr +  0.00 sys =  5.05 CPU) @ 
198137.51/s (n=1000000)
   dirunpk:  2 wallclock secs ( 3.31 usr +  0.00 sys =  3.31 CPU) 
@ 301841.23/s (n=1000000)
     isook:  3 wallclock secs ( 3.08 usr +  0.00 sys =  3.08 CPU) @ 
324886.29/s (n=1000000)
     range:  4 wallclock secs ( 3.23 usr +  0.00 sys =  3.23 CPU) @ 
309214.59/s (n=1000000)
    repeat:  4 wallclock secs ( 3.03 usr +  0.00 sys =  3.03 CPU) @ 
329924.12/s (n=1000000)
    substr:  2 wallclock secs ( 1.22 usr +  0.00 sys =  1.22 CPU) @ 
820344.54/s (n=1000000)
            Rate chkunpk dirunpk   range   isook  repeat  substr
chkunpk 198138/s      --    -34%    -36%    -39%    -40%    -76%
dirunpk 301841/s     52%      --     -2%     -7%     -9%    -63%
range   309215/s     56%      2%      --     -5%     -6%    -62%
isook   324886/s     64%      8%      5%      --     -2%    -60%
repeat  329924/s     67%      9%      7%      2%      --    -60%
substr  820345/s    314%    172%    165%    153%    149%      --
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