Ok, I'm not sure if this is what you mean, but I knocked up an example of the sort of thing I think you're after:
This is basically just a way of capturing a version of $string into the $no_brain variable, but without any 'brain's. It works by taking a copy of $string into $no_brain and then deleting any instances of the word brain from it.
$string = "lalala monkey brain foo bar brain baz";
($no_brain = $string) =~ s/brain//ig;
Tell me if this isn't what you're after..
Hope it helps a bit anyway.