I think that your right in that under some circumstances, Benchmark seems to consistantly favor the first test run. If the code under test does a lot of allocation and deallocation, as I suspect (but don't know) that List::Compare does? Then the first test seems to run substantially quicker. The subsequent tests seem to get a fairly even performance.

Here, the code tested is identical in all cases, but the first test run (Atest), consistantly comes out 23%-46% faster than the other (identical?) tests.

#! perl -slw
use strict;
use Benchmark qw[ cmpthese ];

sub test {
    my @strings = map{ ' ' x 1000 } 1 .. 50_000;
}

cmpthese( 5, {
    Atest => \&test,
    Btest => \&test,
    Ctest => \&test,
    Dtest => \&test,
});

__END__
      s/iter Ctest Btest Dtest Atest
Ctest   9.29    --   -0%   -0%  -31%
Btest   9.29    0%    --   -0%  -31%
Dtest   9.28    0%    0%    --  -31%
Atest   6.36   46%   46%   46%    --

P:\test>373536-2
      s/iter Btest Dtest Ctest Atest
Btest   9.31    --   -0%   -0%  -19%
Dtest   9.29    0%    --    0%  -19%
Ctest   9.29    0%    0%    --  -19%
Atest   7.54   23%   23%   23%    --

      s/iter Ctest Btest Dtest Atest
Ctest   9.29    --   -0%   -0%  -31%
Btest   9.29    0%    --   -0%  -31%
Dtest   9.28    0%    0%    --  -31%
Atest   6.36   46%   46%   46%    --

P:\test>373536-2
      s/iter Btest Dtest Ctest Atest
Btest   9.31    --   -0%   -0%  -19%
Dtest   9.29    0%    --    0%  -19%
Ctest   9.29    0%    0%    --  -19%
Atest   7.54   23%   23%   23%    --
[download]

I have a tentative conclusion for why this might be, but the size of the difference shown by the benchmark seems too big for my thought to explain all of it. So, I'll keep my trap shut for a while and see what others think.