Your test is bad. Your code strings are being evaluated inside of Benchmark, oustside of $X and $Y's scope. Right now, you're using package variables $main::X and $main::Y, which are undefined.

Proof:

use strict;
use warnings;

use Benchmark 'cmpthese';

my $sz      = ( shift || 10 ) - 4;
my $X = 'N' . 'x' x $sz . '000';
my $Y = 'N' . 'x' x $sz . '001';

cmpthese( 1,
          {
           '?!X'  => 'use strict; $X =~ /^N(?!.*00$).*$/',
           '?!Y'  => 'use strict; $Y =~ /^N(?!.*00$).*$/',
           '?<!X' => 'use strict; $X =~ /^N.*(?<!00$)$/',
           '?<!Y' => 'use strict; $Y =~ /^N.*(?<!00$)$/',
           }
        );


__END__
Benchmark: timing 1 iterations of ?!X, ?!Y, ?<!X, ?<!Y...
runloop unable to compile 'use strict; $X =~ /^N(?!.*00$).*$/': Global
+ symbol "$X" requires explicit package name at (eval 2) line 1.

code: sub { for (1 .. 1) { local $_; package main; use strict; $X =~ /
+^N(?!.*00$).*$/;} }
[download]

Fix:

use strict;
use warnings;

use Benchmark 'cmpthese';

my $sz      = ( shift || 10 ) - 4;
my $X = 'N' . 'x' x $sz . '000';
my $Y = 'N' . 'x' x $sz . '001';

print '# input length: ', length( $X ), $/;

cmpthese( -1,
          {
           '?!X'  => sub { scalar $X =~ /^N(?!.*00$).*$/ },
           '?!Y'  => sub { scalar $Y =~ /^N(?!.*00$).*$/ },
           '?<!X' => sub { scalar $X =~ /^N.*(?<!00$)$/  },
           '?<!Y' => sub { scalar $Y =~ /^N.*(?<!00$)$/  },
           }
        );
[download]

The scalar shouldn't make a difference as long as you don't have captures. The real difference is the sub {} instead of ''. The subs now capture over $X and $Y. Using our $X and our $Y instead of my $X and my $Y would also have done the trick.

With the fixed code, it gives me the following, even when reversed:

X = Nxxxxxx000
Y = Nxxxxxx001

         Rate ?<!X  ?!Y  ?!X ?<!Y
?<!X 327712/s   -- -39% -56% -59%
?!Y  535499/s  63%   -- -29% -34%
?!X  750772/s 129%  40%   --  -7%
?<!Y 805492/s 146%  50%   7%   --
[download]