I like making it all into one logical return expression, with no conditionals.

Without the two character fix/spoiler:

sub isValidIP {
   return (shift =~ m/^(\d+)\.(\d+)\.(\d+)\.(\d+)$/ and (($1|$2|$3|$4)
+ < 256));
}
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Update: I wasn't offering a Perl 6 solution, despite the subthread my response was in. That would be

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