Do you the have Perl module enabled in nginx? i.e. Is /etc/nginx/modules-enabled/50-mod-http-perl.conf present?
nginx's master process segfaulting would explain some things. We've had numerous other reports of that module causing segfaults on reload, on Ubuntu servers.
Try disable it, if it's there.
If that doesn't help, it would be handy if you could gdb attach to the nginx master process before it crashes, and provide a backtrace of the segfault.