foreach $ip(@ip) {
   print $out "#" if(/\b$ip\b/i && !/^#/); #only one # at front of line
   print $out $_;
 }

##</code><code>##

 foreach $ip(@ip) {
   last if /^#/; # ALready has a hash . Note - LAST exists this loop
   next unless  /\b$ip\b/i; # No match - so try the next item
   print $out "#";
   last; # Remember to exit loop after one successful match
 }
print $_; # Print onloy once.