In my code was:

sub xx
{
  my ($a,$b)=@_;
# some sentences later
  if ($b =~ /$a/) { ... }
}
[download]

I tried to explain with my sentence:

Please see that "r" =~ // come from "$r" =~ /$r2/ in my real program. And too if I change $r2 to "." if it is empty my program works fine.

I had to use debugger to know what was happened ..

chromatic had the answer !!

Thank you very much, I will use "m//"