I just observe resident memory usage with top or ps.
I tried it on another machine with exactly the same 5.8.8 build and it was not leaking. I wonder if it is a kernel/libc6 issue (the leaker was libc-2.6.1 with 2.6.22 kernel, and the non-leaker was libc-2.3.6 with 2.6.18 kernel).
Another leaker was libc-2.3.2 with 2.6.8 kernel and perl 5.8.4.