I don't think the "fix" used will work. The line is now:

$eng =~ s/\b(?!p)sub/rubber dub/ig;  # thanks to xaco for spotting this

This looks to me like "replace word/non-word boundary followed by something which is 's' and not 'p' followed by 'u' followed by 'b' by 'rubber dub'". This will still replace 'nuclear sub' with 'nuclear rubber dub'. The (?!p) is superfluous because the next character can never be a 'p' anyway, since you say it must match /s/i.

Easiest is probably just to switch the order of the rules, since you can't say "replace the word 'sub' except it if used to be the word 'pub'" :)