sub intermix { 
   my $a_prime = reverse($a);
   my $b_prime = reverse($b);
   $a_prime cmp $b_prime
}

my @list = ('10.1.1.1', '10.1.1.2', '10.2.2.1', '10.2.2.2');
@list = sort intermix @list;

use Data::Dumper;
print Dumper(@list);

__END__
output
======
$VAR1 = '10.1.1.1';
$VAR2 = '10.2.2.1';
$VAR3 = '10.1.1.2';
$VAR4 = '10.2.2.2';