Unless you mention where you get iperf, I assume Optware.
iperf strongly depends on libuclibc++ and don't use any 2.6 specific syscalls. It can be easy compiled by our toolchain:
I can attach ready binary which requires libstdc++.so.6 & uClibc 0.9.30 from wl500g googlecode toolchain, but since Optware uses same library names, you have to provide proper library paths with LD_LIBRARY_PATH or /etc/ld.so.conf for binary (libstdc++.so.6 should be taken from hndtools-mipsel-uclibc-4.3.5-1.tar.bz2).
./configure --host=mipsel-linux --prefix=/opt ac_cv_func_malloc_0_nonnull=yes