$str =~ m/^#ifdef\s+($macro)$(.*?)(^#else$)?(.*?)^#endif$/sm;

Combining non-greedy matching with a non-capturing group on the conditional clause will do you:

$str =~ m/^#ifdef\s+($macro)$(.*?)(?:(^#else$)(.*?))?^#endif$/sm;

1. Start with the required item #ifdef $macro with variable whitespace on its own line.
2. Grab the shortest set of characters that still supports the regex.
3. If an #else is encountered on its own line, use that to delimit a break and grab text following it into the next buffer.
4. End with the required item #endif on its own line

Note that the lines will still be surrounded by newline characters, as in your original case. To get rid of those, you could instead use:

^#ifdef\s+(ABC)\s+^(.*?)(?:(^#else)\s+^(.*?))?^#endif\s*$

Note ikegami's point above re: nesting - regular expressions are terrible about dealing with nested structures.