I have the same problem. Since I do not even have an uncommented
cache_mem line in my squid.conf, it is obvious the error message is
bogus, and cache_mem probably comes from the default in
src/cf_parser.h (default_line("cache_mem 8 MB")).

I cannot find a solution myself, sorry.

% sudo squid -z
FATAL: Bungled Default Configuration line 9: cache_mem 8 MB
Squid Cache (Version 2.5.STABLE5): Terminated abnormally.
CPU Usage: 0.050 seconds = 0.010 user + 0.040 sys
Maximum Resident Size: 0 KB
Page faults with physical i/o: 0
zsh: abort (core dumped)  sudo squid -z

Squid 2.5.5nb5 compiled from the package source. NetBSD 1.6.2.