I think it's not going to make that big of a difference to have it check if the $regex variable is there or not. But if it bothers you, by looking at your code I think you can just do

    sub execute
    {
          return unless $regex;
          # ...and then do the rest
    }
[download]

As a sde note, if $regex never changes within the scope of execute(), you might want to optimize how the regex is treated, instead of worrying about conditionals

Something like:

    sub execute
    {
          my $compiled = qr/$regex/;
          .....
          for...{
                nex if $host !~ /$compiled/;
          }
    }
[download]