If you get next message
Can't locate object method "new" via package "LWP::UserAgent" at /usr/share/munin/plugins/nginx_request line 106.
when you are installing nginx plugins for munin in Debian, this means that you don’t have perl LWP library. Installing it fixes an issue:
apt-get install liblwp-useragent-determined-perl
After munin-mode restart munin will start to create graphs.