knoppix@Microknoppix:~$ perl -E '
> $line = q{(a) Line 1. b) Line 2. (c) Line 32. d) Line 42.};
> @arr  = split m{ (?<! \( ) (?= \(? [a-z] \) ) }x, $line;
> say qq{>$_<} for @arr;'
>(a) Line 1. <
>b) Line 2. <
>(c) Line 32. <
>d) Line 42.<
knoppix@Microknoppix:~$