hum, it works for me...

while (<DATA>) {
   if (length($_) > 10) {
      if ($_ =~ (m/192\.168\./g)) {
         $readlines++;
      }
   }
}

print($readlines, $/);
# prints 3, although you're probably expecting it to be 2.


__DATA__
192.168.0.1
1.192.168.0
12345678901
dfhasfhasdjk
192.168.33.4
[download]

The /g is unecessary, misleading, and a potential cause of problem. The $_ is the default argument for both length and m//, so it can be omitted:

while (<DATA>) {
   if (length && /192\.168\./) {
      $readlines++;
   }
}

print($readlines, $/);
# prints 3, although you're probably expecting it to be 2.


__DATA__
192.168.0.1
1.192.168.0
12345678901
dfhasfhasdjk
192.168.33.4
[download]