In theory, can_read() is more efficient in cases where you have a large number of clients that remains mostly constant between calls to can_read(), since add() and remove() should be performing only bit operations, wheras IO::Select::select() re-creates the bit mask every time.

In practice, I wouldn't be able to guess which one is faster. Graham Barr (the author of IO::Select) does implement add() and remove() using bit operations, however, he has a lot of rather inefficient code between add(), remove() and the code that actually does the bit operations.

Personally, I make the bit fields myself. The code isn't that complicated, and I handle things that can_read() does not such EINTR. add() is implemented in terms of vec(...) = 1, etc.