Quoting the man page of getdents:

These are not the interfaces you are interested in. Look at readdir(3) for the POSIX-conforming C library interface.

That's all that you need to know about getdents(2). Perl has a readdir function that calls readdir(3) internally, and I'm quite sure it is optimized. Readdir(3) itself is most likely implemented in the libc as calling getdents(2), with a fallback to readdir(2) for older kernels.

I'm looking for a fast way to list the contents of a directory (with thousands of files) on Linux by using Perl.

opendir, readdir, closedir. Benchmark that. Compare with ls. Most likely, you won't get faster than that, simply because perl has higher startup costs and does not run native code, but instead follows a complex data structure representing your perl script.

My guess is that the bottleneck is the disk and its interface, not the actual functions called to read the directory. Sure, libc and perl add some overhead, but not that much.

Alexander