sub isValidIP {
   return (shift =~ m/^(\d+)\.(\d+)\.(\d+)\.(\d+)$/ and (($1|$2|$3|$4) < 256));
}

##</code><code>##

   return (shift =~ m/^(\d+)\.(\d+)\.(\d+)\.(\d+)$/ and not (($1|$2|$3|$4) > 255));